I'm using metric measurement and waterbased inks, so I can't really help you on specifics, sorry. I
think you want around a 110, but that is from memory and could be completely wrong.
There are a few ways you can find out though:
1) Pickup a quart of Plastisol and read the label. Most inks I've seen give a recommended mesh count (it might not, but no harm checking).
2) Check a manufacturer's website and see if they give recommendations there.
3) Look at a screen supplier's website - they may only offer one or two mesh counts (which would give you a good hint

) or they may state a recommendation.
4) Go to your screen supplier in person and simply ask them. It should be an easy/common question for them and they should have no problem helping you out.
5) Google
6) Wait for someone who knows more to post here
When I first went to my local screen print supply shop to talk to them, they ran over a few of their products for me, and pointed out I would need 43t mesh for textiles and 90t mesh for paper printing. In my case I already knew that, but at least it confirmed for me that they were giving me good advice

When I went to a smaller art supply store they only stocked three mesh counts (the proper supplier has fifteen), so with them you would have a two in three chance of picking a reasonable option (or you could ask the guy behind the counter for advice

).
If you have a good supplier you should be able to trust them. If they tell you you need ten different meshes they're gouging you, but if you say something along the lines of "I'm new, I'm printing on t-shirts, I don't really know what kind of screens I need - can you please tell me?" they should give you the answers and not make you feel like you're asking stupid questions (you're not).
I asked some pretty stupid questions of my supplier, and I got good answers. They're there to help, because that means sales for them.