It depends on a variety of factors - number of follicles, size of follicles, age of mother, fertility issue, method (IUI, IVF, natural),
Estradiol, type of medication - Clomid
vs. injectables, etc. My impression is that if using IUI, more follicles increase your chance of getting pg with at least 1 slightly - if 4+ then ~ 22%. If you get pg, it increases your chance of higher order multiples (3+) slightly as well, but the chance of higher order multiples is still small - < 5%.