I'd lean toward whatever you feel you can reasonably handle, as the sole mod. Not just in the sense of more players = more busywork on your end, but because it will most likely fall to you to resolve any player conflicts that may arise. I just think you should be able to enjoy your own sandbox, it shouldn't become a second job.

But I will say part of the draw for me when Fox was telling me about this little sandbox is that it's invite-only. In my experience, that's been an effective method of ensuring that, for the most part, all the players will generally gel pretty well with each other and it creates a more welcoming environment than I've experienced in bigger public games; everyone is able to be "heard". Completely opening it up might run the risk of disrupting some of that, depending on what kind of interest it would generate and who might join. And maybe it'd be totally fine and we'd just gain a bunch of new awesome players! As long as you feel you can handle the playerbase growth and any potential issues in a mod capacity.

I guess those are my two main concerns. Making sure you don't hit burnout because the sandbox explodes, and weeding out those Please Don't Make The Mod Create Rules For This Game Just Because Of You sort of players.