relay ratings at 120v and 240v...can I use a 20A 240V relay with a 13A 120V pump?
NO20A/NC10A is one rating for the normally open connection 'NO' and one for the normally closed connection 'NC'.

If you're only using the NO connection then you don't need to worry about the NC rating.  (And you don't need a changeover relay in the first place, but that's a minor issue)

I can't see that NO20A/NC10A bit in the datasheet I'm reading from that page, so I don't know if I'm looking at the same relay.  Generally, if the relay's OK at 240VAC it will have no worse a rating at 120VAC.

There are often two ratings quoted, one for resistive loads and one (much lower) for motor loads.