I haven't applied/been admitted to optometry school, so I may not be the best person to answer this question. However, I've read on this forum and heard from students that there are a couple of things you want to get from shadowing. One is that you will have to shadow with one doctor for at least 40 hours in order for this doctor to write a recommendation on your behalf. The second, and possibly more important factor, is to shadow in a variety of settings. I've heard about a few people who had spent more than enough hours shadowing but it wasn't enough. Schools want you to get exposure to different settings--examples being private practice, hospitals, lasik centers etc. In total, I'm guessing that shadowing with 3 optometrists would be sufficient and 2 MIGHT get you by.