You ideally need two, but you may end up wanting three, though in a pinch you can get away with one.
With one you will always have to be on top of charging, not great when you need to marshal etc, and you won't always get it toped up in time if it's not a club/meet with numbers to spread out the heats.
With two you can get by ok, odds are you won't be left waiting unless you forget something, though your batteries still won't get a chance to rest as such, one will likely be charging and the other ready to go in the car.
With three, it's golden, forget or be lazy and you still won't be left hanging, better yet a pack can rest after it's been used (which in theory is good for them).
As for kv, well it's different with all the brands, if you need to know the kv of a motor from brand X, you will have to go web surfing to their site and check the specs, for example I had a SP 10.5T and it was 3800kv I think, then got a Hobbywing and it's 3300kv, so no chart will likely exist.