I have no idea how many friends I really have, but I think there are fewer friendsit's on the fewer side unfortunately. I have no idea how many friends I really have, but I think it's on the fewer side unfortunately.

The expression is "on the [adjective/adverb] side". You can think of it as describing a continuous scale: here, at one end of the scale we have "many", and on the other we have "few".

I have no idea how many friends I really have, but I think there are fewernot many friends unfortunately. I have no idea how many friends I really have, but I think there are not many friends unfortunately.

"few/fewer" wouldn't sound natural in this context but "not many" has similar meaning

I have no idea how many friends I really have, but I think unfortunately there are only a fewer friends unfortunately. I have no idea how many friends I really have, but I think unfortunately there are only a few friends.

I have no idea how many friends I really have, but I(I) think there are few truer friends (unfortunately). I have no idea how many friends I really have, but (I) think there are few true friends (unfortunately).

NThe number of meetings with school friends haves reduced as I get older. The number of meetings with school friends has reduced as I get older.

NThe number of meetings with school friends haves reduced as I get older. The number of meetings with school friends has reduced as I get older.

The subject of the sentence here is "number (of meetings)", which is singular. Hence, "has" is expected.

NThe number of meetings with school friends have reduced as I get older. The number of meetings with school friends have reduced as I get older.

just needed a definite article

Number of meetings with school friends have reduced as I get olderMy school alumni meetings have reduced in recent years. My school alumni meetings have reduced in recent years.

I want some friends who has same hobby seriously. I want some friends who has same hobby.

The seriously just doesn't fit here.

I want some friends who hasve the same hobby seriouslyas I do. I want some friends who have the same hobby as I do.

(1) "Friends" is plural, so "have" is used. (2) Since you've not brought up your hobby before this, it might not be immediately clear that by "same hobby" you're comparing said friends to yourself. The sentence might be interpreted to be referring to friends who share the same hobby between themselves but not with you. "As I do" clarifies this point. (3) Perhaps what you meant to say with "seriously" is this: "I want some friends who pursue the same hobby I have seriously." "Seriously" is an adverb, and something like "pursue" might be the verb you're looking for.

I want some friends who hasve the same hobby seriouslyas me. I want some friends who have the same hobby as me.

I want find some friends who hasare passionate the same hobby seriouslyas me. I want find some friends who are passionate the same hobby as me.

QUESTION: What hobbies are you passionate about?