A single herpstat 2 or a single herpstat 4 would be fine. The herpstat 2 is literally 2 herpstat 1's in a single box, and the herpstat 4 = 4 herpstat 1's in a single box.
What you want is 1 probe per heat source. A herpstat 1 has 1 probe. A herpstat 2 has 2 probes so it can run 2 heat sources.
A single herpstat 4 can safely control 4 heat sources.