The range I go to most often limits you to 4 guns per range trip. I find that usually that is a reasonable number to get some meaningful practice and also get some variety. For instance, for tomorrow's range trip, I'm bringing my CZ P01, my S&W M&P40c, my 3" Rossi 461, and my S&W 1911SC.
The CZ P01 is going to be one of my carry guns. After this trip it will finally have enough rounds for me to put it into my carry/HD "rotation" (I don't rotate, they are more a range of selections filling slightly difference niches). I'm bringing 50 rounds of Fiocchi 124gr JHP, 75 rounds of defensive JHP (50 rounds of 124gr XTP Hornady American Gunner, 25 rounds of 115gr Hornady Critical Defense), and 100 rounds of 115gr FMJ (1 box of Federal, 1 box of S&B).
The 1911SC is being returned to my carry/HD selection as I shoot CZs and 1911s better than anything else. I'm returning to it coming from DA first shot/DAO pistols for defense previously, so it needs some re-familiarization time. You see, I don't shoot as much as I used to, and my 2 most used carry/HD guns are usually the first to come to the range (and get the most ammo) so it wasn't shot as much as other guns the past few years. It is shooting 10 rounds of Magtech 185gr JHP .45+P, 20 rounds of 185gr XTP Hornady American Gunner, 50 rounds of Fiocchi 230gr JHP, and 50 rounds of Magtech 230gr FMC. I already know it is reliable as it used to be one of my carry/HD guns, but I need some refamiliarization.
The S&W M&P40c is brand new and getting its first range trip. I need to see if it is reliable enough to become one of my defensive guns. I'm bringing 100 rounds of some old Federal 180gr JHP (I may bump it to 150 rounds, or buy a box of FMJ before I go).
The Rossi 461 was bought to be a defensive gun, but due to issues with my 2" Rossi 461 I have never really trusted either the 2" which gave me trouble or this 3" (nor have I shot the 3" much). It is getting one box of S&B 158gr FMJ .357mag.
If I had fewer guns I felt the need to test or familiarize myself with, I might just take two defensive guns and a couple guns for fun. The defensive guns would get 100-200 rounds each and the fun guns would get 50-100. Other times, I might just take one or two guns (at least one will always be one of my defensive guns), at which time each gun may get more rounds (sometimes significantly more rounds).
What do I start with? I don't really have a "system", I start with whatever I'm in the mood for when I get there usually. Which I spend the most time with is usually a 9mm (cheaper ammo) and when I had a .22 handgun I did shoot it a lot (when it came with me) due to cheaper ammo so I usually had more of it.