I have had difficulty thinking what I write in English recently because I stopped to hang out with my friends after the pandemic spread across a large region of my country/city/the world.

The sentence is technically correct, but region is a very general term not often used alone - it would be better to describe what kind of region.

It is certain that some of my friends have already got married or workmoved away from here for work.

this is another way to say the same idea.

I have had difficulty thinking about what I write in English recently because I stopped to hahanging out with my friends after the pandemic spread across a large region.

I would also replace "what I write" with "what to write," but the meaning wouldn't be the same as what you currently have in English.

A while ago, I heard that the amount of your friendnumber of friends you have reaches its peak at 25 years old.

"Amount" is more for uncountable objects.

It is certain that some of my friends have already gotten married or worked away from here.

"Work away from here" is fine, but I think something like "started working elsewhere" ("started" to indicate a change) would be better.

These situations are totally different from my college days.

I think this is grammatical, but a more natural way to phrase might be:
- This situation is totally different from how my college days were.
- These situations are totally different from those that I encountered in my college days.
I'm not completely sure, but I think it sounded unnatural because the comparison wasn't fully written out.

I don't want to count how many friends I will have eventually.

If the self-restraint wouldsocial distancing continues for a couple of years, how dowill I spend my 20s?

I'm not completely sure if "social distancing" is the right translation for 自粛, but based on context, it's what makes sense to me.

"Would" typically doesn't appear in "if" clauses, outside of set phrases "if you would be so kind," "if you would only," etc.