Letrozole/trigger shot

Hey all! I’m on my first round of letrozole. I went in for my first follicular scan yesterday my biggest follicle was 15.9 and I go in for my second scan tomorrow. My dr talked about a trigger shot but he says it all depends on how much my follicles grow in the next two days. What am I hoping for? Does a follicle need to be at least 20mm before I get that trigger shot? I’m kinda confused about this process... he also says I mite need to up my dose next month. But he seems so confident about everything...I’m so lost on what I should hoping for.