43 Mechbays, all in use. 38 Mechs are on master level while STK-5S & Misery as well as CTF-4X, AWS-9M and LCT-1V are not completed yet. Mastered some other mechs as well, but sold them for freeing up mechbays. Of those 43 mechs I use about 2/3 quite regularly... depending on my mood or the drop composition of my group.
Commando mastered, I love blasting around in full speed TDK. And I have my 2D for when I want to shoot people in the back. Catapult Mastered. Run around with my Jester when I want to be a fast Heavy. K2 when I want to be a complete Douchebag with a pair of 20's. Cataphract Mastered: Ilya Mourmets, 3x UAC/5 and a pair of med pulses get me a pretty fair way when I just feel like ripping people up. Always a delight when you get 4-5 salvos out without a JAM. I have the Hunchies working now: Not very enjoyable, but want to see how they fair once elited. Haven't played any assaults yet. I don't like being slow.
Jenner, Spider, Raven, Centurion, Cataphract, Jager, Atlas. All variants excluding the Boars Head. I have a few more mastered, but I will need to complete a few variant mechs before then.
Being a assault - heavy pilot, and a mostly retired test pilot I might be throwing off your curve. I have mastered every assault chassis, and keep at least one mastered of every other category on the way down. The lighter it gets, the less amount of mechs I have. I have only one light, my Spider. I have two mediums. Shadowhawk and Centurion I have one heavy current. My Catapult. I've mastered the Phract and the Orion but didn't like either. All of the assaults, usually two of each type as well kept.
Eight Mechbays total (i'll probably be buying another one or two with leftover MC soon). I've mastered the Commando, Cicada, Blackjack, Orion, and Awesome. Out of those guys, I've held onto no Commandos (sold Streakmando to get the bay back a while ago), just the X-5, a BJ-1 and -1X, just the Protector, and just the AWS-8T. I've also got a Dragon whatever the one with 2/2/2 hardpoints is (elited and sold the other two, haven't had the desire to finish eliting this one) and a shiny new Firebrand, so one free bay.