How about going fishing?
You could ask things like,
How many relationships are too many?
If they answer in a hypothetically beneficial manner then you could proceed with something like,
How about you? What's too many for you?
But that's just MHO.
I'm almost too blunt for poly it seems sometimes.
It's just me, my ramblings and the elevator music in my head.
Me: bi, reformed wild-child turned mom and house maid LOL
Crux: straight/hypersexual, possibly mono?
BC: our son, 5 years
CM: second son 9 months.
Mouse: girlfriend! Status new, feelings not so much.