I don't know where you heard it but that is not true. The current carrying conductors must be the same size. What you might be misunderstanding that in some cases the EQUIPMENT GROUNDING conductor (i.e., the safety ground not the return from the motor) can be a smaller size.

For 15 and 20A circuits however, it still has to be 14 and 12G respectively. It's only when you get up to larger sized circuits that it is possible that it is legally smaller.

The number of pumps you can put on a circuit is not regulated. It's determined by the load (amps drawn/size of the motors) and the nature of their operation. etc... You'll need to give us much more description as to what you're trying to do to give an answer on that one.

Some old codes allowed neutrals on multi-wire circuits to be smaller, but that was decades ago.

But only in the case where there were 240v loads on the circuit that precluded the neutral from being overloaded with a untoward balance of 120v loads. That doesn't fit the description here anyway.