How many home runs can share neutral

Depends on how many 3 conductor wires you are planning to use. 1 hot on each bussbar and a common neutral yields 2 circuits for each 3 conductor wire. Or do I misunderstand your question?

A single phase system will allow two phase conductors to share a neutral, 3 phase would be three phase conductors.

Sharing neutrals is not recommended in a well designed system. An open or broken neutral will cause abnormally high voltages to be introduced into the circuits and cause damage.