I think I've read about guys using just one pac and charging between races. Lipo can be charged as often as you need to, unlike NiMh. You might want a spare, but get some advice from guys at your track that are running lipo now on what is working for them.
1/8ths usually run 4S and I'm starting to see a shift to higher cells counts and lower kv motors to take advantage of higher voltage, lower current (less heat) operation. If you get a pr of 2S you could run them together with a series harness and they would be equiv to a single 4S (but weigh more and be physically larger than a single pac).