Just about every publication covering college football seem to put out their own All-American team of some sort. According to good ol wikipedia there’s five lists of record and if you get on any of them you are an All-American.
As far as how many, each group usually selects an actual team, just like you would see on the field. So 11 on offense, 11 on defense. Some release a second and third team as well, so someone can be a “Second Team All-American”.
Big name schools traditionally get more play and notice nationally, so they’ll usually have more All-Americans. Stats will help, but being on a big winner seems to help more.