Follicles, IUI, and Letrozole

Had my first follicular ultrasound today after taking letrozole. Expected a couple mature follicles, but only had one small one (13 mm). They are having us come back in a few days in hopes it will grow some more. Does this lower our success rate to only have one, and it not be as large as we had hoped, or is that irrelevant? Not sure if this is the norm.