Q:

Is it normal to have 12 activated Thin Men on month 3 on Impossible difficulty?

I can't remember map name, but it was a small building in center of map. I think it was a pizza shop. I only had ballistic weapons, but was 1 day away from lasers. Did detour over xeno and containment. My goal was to capture a sectoid. I moved on blue move from start. I activated a 3 thin man pod. Ok no problem. Sniper missed. Mec missed. Run and gun my assault to door of shop to get at least one, but I activated 2 more pods of thin man inside the shop, along with one more pod of thin man on the right side. Sniper dies, before he had time to move. 
Now I am facing all 12 enemies of that map, all thin man. Luckily I was able to revive my sniper, but my assault ended up  dying. 
Is fighting this many Thin Men at once bad luck, or normal for first mission in May on Impossible?

A:

On Impossible difficulty coming across a mission with 12 enemies isn't uncommon, and depending on the mission type you're essentially guaranteed to have at least 12 enemies per missions. Having all those enemies being Thin Men is probably a tad on the unlucky side, but on Impossible difficulty there's no limit to how many of a single alien type can appear on a mission. The UFOpaedia  has a page which goes over Alien Deployment in detail, although it can honestly be really confusing to read. The fandom XCOM wiki has a page which goes over difficulty levels, which has some details on pods as well.
As for your question of having all these pods activated at once, well, that sounds like it's a combination of bad luck and just poor tactics. When fighting squads of enemies you always need to be really careful to avoid running into unknown territory, since as you discovered, there's the very real possibility of activating additional pods. This is even more of a problem on Impossible, since on lower difficulties the game will try to move pods in such to reduce the chance of you activating multiple pods, but on Impossible the game stops doing this. 
So the best way to avoid this problem in this future is to just switch up your tactics. You should always try to avoid running into an unexplored part of the map once you're already engaged in combat. Instead, you should actually try to fight the enemy fairly close to where you activated them. You can also try to lure them back a bit, further into areas you've already explored, since that'll allow you to flank them without worrying about activating more pods.

