Hi there Fiona,
I'm Jon, a Shopify Guru.
I'm going to move this post to the Shopify Discussion section, where you're more likely to get more answers.
But for now, there are a number of ways you can sort your products in collections. You can sort them manually, so you can sort them yourself into which ones have the most availability.
Or, you can also setup an automatic collection to hide products that are sold out. This guide goes through how to set that up!
I hope that helps! All the best!