So with the new V5 system coming out. Which “legacy” (I think that’s what they’re called) sensors will be permitted and which sensors if any won’t be allowed for use in competition?

As far as I know all of the old sensors that were legal last year are legal with V5.

How many ports will be available for sensors with V5?

While there are already 8 three-wire ports, you can get the “3-Wire Expander” to put 8 legacy sensors into a single smart port. I don’t see the “3-Wire Expander” on VEX’s site for pre-order yet, though.