I only had one follie, 20 mm, and the IUI worked! Got my bfp yesterday.
I've heard that it's the quality, not the quantity that's important. 20 mm is a good size, and if you're doing it at the right time and have a good lining, that's what you want! Good luck.
I had only one follie the cycle I got pregnant with this baby. I thought there was no way I had a chance... well, it turns out it really only does take one.