Yes , If you know somebody and they also know you then you can send friend request to them . But please don’t send friend request to anybody if you are unknown for them . If they complaint to facebook then your facebook account can be blocked.
So question is that how people make thousand of friends. Actually facebook is useful for may purposes and before sending friend request to anybody you should know how they are using facebook . If somebody have very few friend means they are interested in family and known friends and if they receive unknown friend request then either will reject it or they can complaint to facebook .
If people are interested to make unknown friend they are active user of facebook and they take part in comment , likes of others post and photos and automatically they make new friends on facebook.
So send the friend request only to those facebook user who are interested to become your friend. Don’t send friend request to unknown facebook user otherwise you can be block on facebook
But for those who are not interested in unknown friends request , please take it easy and just reply them that you are not interested.