Typically at least two and no more than six. If I'm traveling, it might get down to one. Today it's five.
Except that I cheat. I don't count several Pilot Varsities which sit around half forgotten until I pick one up on a whim, and find that it still starts right up after several months.
Returning to my much earlier reply, right now it's four. Again not counting the Varsities. And again not counting three Esterbrook Dip-Less desk pens, which are a sort of dip/fountain pen hybrid and have ink always in the wells.
When some silicone sacs that I ordered come in tomorrow, I'll be putting one in a vintage celluloid pen that needs one, and then of course I'll want to fill it, so that will make five if I don't write something dry before then.
Again, not counting the Varsities and the Dip-Less sets. Oh, and I guess I have cartridges in four Pilot Parallels that I rarely use. How often do you need a 6.0 mm nib? But they don't dry up any faster than the Varsities.
What made me think of this is that I'm trying to make my journal a bit less discursive, meaning fewer pages a day, meaning it will take even longer to empty a pen. So I really should consider cutting back on the inked pens. But as long as my pens aren't drying out, I don't really worry about it.