What does the number after a machine learning model name mean? I'm not sure if this is off-topic, but I'm posting here anyway.

So I saw lots of machine learning models have like an ID after their names, for example, resnet101, resnet152, densenet201 etc. What exactly do those numbers 101, 152 and 201 mean? And how it's determined?
Sometimes it refers to a version (like windows 10), sometimes it refers to a size of the parameters, like the size of a layer, the number of parameters (like for GPT models), or the size of parameters in memory. They are just names so we can know what we are talking about. I don't think there is a general convention.