I've been downsizing / upgrading as well over the course of this year and am keeping 11 purses (after my last purchase arrives), 4 clutches & 5 SLGs. Unless something new is dramatically different from something I already own, I've been good at the 1 in / 1 out policy
To help stay on track, I wrote out a list of all my purses, colors, sizes, types of handles, hardware & material in my iPhone notepad. That way if I'm tempted while out shopping I can see my list and determine "nope, already have one" or "what do I have to get rid of to make room for this" & then decide whether I'm prepared to do that. Helps tremendously with impulse purchases!