What thickness of closed cell spray foam should I use on my basement walls?
Closed cell foam typically has an R6 value (that's R6 per inch)

http://www.fomo.com/resources/technical-bulletins/opencellvsclosed.aspx

So 1" of foam would be R6.
2" of foam would be R12

I've lived near you - Wisconsin gets cold and stays cold - and in a basement the longer the ground stays cold the more thermal break you want. so frankly in your climate, I'd be looking for foam to completely fill between the joists with foam - 3.5" if you're using typical 2x4's - giving you closer to an R20.