Can I Have More Than 32 Maps/Can I Create Both A Good SP And DM Map

You have multiple methods of making a continous mapset with more than 32 maps...

MAPINFO/ZMAPINFO for ZDoom-compatible ports,

UMAPINFO for "boom-compatible" ports (It works in gzdoom and this special version of PRBoom+, but not much else currently.),

And finally EMAPINFO for Eternity Engine.

I wouldn't recommend making even a 32 map long megawad on your own though... You'll easily burn yourself out if you do that, seriously.

If you still want to make a mapset, stay small. 10 maps at most, and less if you feel burnt out.

In regards to your question about deathmatch maps, big maps suck for deathmatch. It can take ages for two players to meet, and then the deathmatch gets drawn out because of it.

What you can do, however, is to put deathmatch spawns in a separate, otherwise inaccessible area. That way, it actually plays well. It could even be a small chunk of the real map if you want the deathmatch map to feel like it actually takes place in the singleplayer map.

Interconnected maps give lots of routes, which tends to work well with deathmatch, as it gives opportunities to do flanking and the like.

Lots of deathmatch starts just means there's just a bunch of places you could theoretically respawn in. It doesn't imply the amount of people deathmatching, and more than even 4 players joining a single deathmatch server at the same time is kind of hard without asking people to join.

I wonder if I have misunderstood what you meant by big, but usually the later maps in each episode of doom are worse due to their size and thus generally dull deathmatch gameplay. If we're going by the size of, say, E1M7 or E2M7, it would absolutely suck for deathmatch, let alone anything bigger. E1M1, on the other hand is pretty small and works pretty well for deathmatch due to the secret switch at the start(exclusive to ultimate doom).

If there's a lot of players, a big deathmatch map could work, but that's sadly a big if. Even if you had bots on it wouldn't work out that well since they usually get stuck on stuff, meaning they're basically worthless as opponents.

You need to be really confident on your mapping skills, if you want to make more that 32 maps. People don't usually play 32-map megawads, especially if someone says it's their first ever wad and contains their first ever map. What I'm trying to say is, you need to start small. Don't you ever make the mistake of building a 32-map megawad. I made the mistake and my first ever released maps were a 33-map megawad, which no one liked in the first place. 

1- It can be done via MAPINFO, where you can also modify map names, par times, skies, etc. 

This is what it usually looks like:

map MAP01 "My cool map's cool name"
{
    LevelNum = 01
    skybox = SKY1 (can be any sky from SKY1-SKY3 on Doom 2, or sky1-sky4 on Ultimate Doom. If you have any custom skies, you can name them sky5, and put theme here)
    next = "MAP02" (indicates which level comes next)
    sky1 = "SKY1", 0 (as applied before, can be any sky or custom sky)
    music = "D_MYMUS" (can be the name of the track you assigned to it)
    par = 60 (Type  in the time in seconds
}

2- In regards of your second question, a map that I made is dummy big and dummy non-linear (there are 2 paths, one of which doesn't even require the red key, but is otherwise longer as a path), but still has a decent deathmatch arena, because it takes part in a somewhat wide-open area, where, in singleplayer, the player was going to get crushed, but in deathmatch, the crushers aren't even triggered, so it's possible to pull it off in one area.

Question is how you'd divide those maps, since you'd be cannibalizing other maps to have multiple separate playable deathmatch chunks, or making a deathmatch version of the wad. Although at that point you might as well just make a mapset that's designed for deathmatch only.

That final question's answer is probably a yes, I wouldn't be surprised if that's been in done in a lot of deathmatch wads already, to be honest. How well it'll work depends on the size of that large area.

Something to remember is that a lot of maps and mapsets get tested by other people before they are fully released. A map that has been playtested by other people and polished from the feedback usually feels better than one that hasn't. This is especially true in deathmatch where the players are a major part of the gameplay.

First off all, thanks for replying, Alper002!

Now, in relation to my first question, now I do remember just browsing for famous megawads on Doom Wiki.org and noticing that most of them were made by several people. How I did not remember this, is beyond me...

About the second one, yeah, you are right about it, too. But it got me thinking: and if I put more player spawns than usual, could it make the map work, since there is a bigger number of players? And even if it must be small, could I still make it non-linear and make the various areas interconnected?

Thank you too for answering my questions, Elio Nako! Oh, and don't worry, I do not plan on making a 32 map megawad immediately. That question was just out of my curiosity.

Thanks again for helping out the newbie I am, Alper!

And no, you didn't misunderstood me about big maps, that is what I meant to say: levels with a very big size. But now I understand that it is not exactly the best idea to make arenas that huge...

But something you said earlier got me thinking... What about, in a megawad, have the smaller singleplayer maps be used themselves for DM, but have the larger maps divided by their recognizable areas and landmarks? It still would not waste a single room from the original levels and make it feel like you are playing the map, while improving the experience of trying to destroy your opponents, since the arenas are smaller. Not to mention that it would increase the amont of DM maps itself!

Finally, just another question. Can I build a map that is relatively small, but that does contain quite a big sized open area, with, for example, a rocket launcher, plasma gun or BFG, in order to create a risk reward system, in which you will be open to damage to get the powerful weapon?

Alright then, I have all my questions answered.  Thank you all for helping me out!