Typically, one follicle becomes dominant and that is the one that releases the egg. If I remember correctly, they like to see it reach 17/19mm (I can't remember..but it was something like that) before they anticipate it to release the egg.
It would depend on the size of the follicle. You can see them early on (day 6 or 7 of your cycle) but unless they are 18mm or larger you aren't ready to ovulate.