I've completely given up on this definition - I'm down to 'maybes'.
Maybe if you have more than one or two guitars you have a collection, whether you call it that or not. On the other hand, two guitars could be a collection. Perhaps it depends on intent?
My own thinking is that if you're getting guitars to have a guitar collection, you have a collection. If you're getting them to just play, maybe you're still the owner of a collection, it's just a different kind of collection.
I have 5 electric guitars that I use in my work. Whatever that is, it is. If I'm a collector, well, so be it. Perhaps I've mellowed in my dotage.