You receive one save-game slot when you play on RPG Solo, with no theoretical limit(?) depending on your donations, but how many simultaneous games do you feel comfortable running for yourself?
Since I track and write everything in Google Docs and only use the RPGSolo tool to generate information, I've never had the need to purchase additional game slots.
As far as playing games go, not counting quick little one-shots like a Quill letter, I typically only play/write one campaign at a time. I can possibly claim two at once since I did run a short three-episode game in the middle of a longer campaign for another game, however, even then I completely set the first game aside while playing the second. So I was exclusively focusing on only one game at a time.
My imagination tends to flutter around, so keeping oriented on a single thing is often difficult. I can knuckle down and do it when I'm working on something in my real life, but when I'm gaming I have a hard time ignoring a cool idea I might get.
Though I guess this may be one of the advantages of solo play. Since there's no need to keep a group of players happy, a GM can do whatever he or she likes. Want to play something different this week from last week? Go for it. No one's going to complain. Want to hop back and forth between two or three games? Likewise.
At the same time, I know that dividing my attention can sometimes result in neither thing coming to completion, so I have to watch it.
I have 13 slots. I don't play all those at once, and I too use google docs to hold my information. It's just easier to edit information there. I too use the game slot to only generate rolls and oracle answers. So, it's not technically necessary to have those game slots, but I wanted to support the site and its author. I'm mainly just running one game at the moment (my Ironsworn one).