Here's my two cents about this...
The only time you'd want to specify the P2 selectors is when using policy-based IPsec VPN on one side or both. For route-based IPsec VPN on both sides leave them at 0.0.0.0/0. Using P2 selectors on route-based IPsec VPN doesn't add anything other than complexity.
If you do have policy-based IPSec VPN on one or both sides, you'd want to consider how you want the routing to be. If you route a big subnet into the tunnel that can potentially create issues further down the line, so you'd want to keep it as small as possible, but also as few P2s as possible. If you have two /32s in the same /24, and routing that into the tunnel won't create any issues, sure, do that. But if you have two /32s far apart (requiring /22, /19, etc) it would probably be better to have two individual P2s. Both sides need to agree on what the selectors should look like, so there isn't really a "best practice" that always applies. Sometimes you end up with a bunch of P2s, other times you can get by with one.
Hope my answer helped!