Is there any limit for blinking sectors?

In vanilla DOOM mapping lightlevels ( max or min lightlevel ) depend on the brightness of the neighboring sectors.

Is there a limit, probably 255 sectors.

https://doomwiki.org/wiki/Lighting_effects

In G/ZDOOM you can use action special 113 to specify the range of light variance for each sector.

Is there a limit, probably 65535 sectors.

https://zdoom.org/wiki/Light_Flicker

Yeah, the upper brightness limit is 255.  That is as bright as a sector will go in Doom, vanilla or otherwise.

The way you make it work, is you set the sector you want to flicker to the brightest light level you want it to have.  Then when it flickers, it will flick down to the light level (presumably lower) of the sector you place your flickering sector next to.

Say the room you want the flickering sector to be in, has a brightness of 104.  You want your flickering sector to flicker really brightly.  You set it to 255 then set the sector effect 'flicker' to it.  And wallah, you have a flickering sector.

Note, your flickering sector must be brighter than the sectors around it.  If it is darker, it won't work properly.