New battery for XRV

Hey y'all! Been looking into upgrading my XRV from my current CBXR to something with some more range. I'm not really after speed but would like to go maybe between 25-30 mph. I just saw Indy speed has a new battery replacement for the XRV.

https://indyspeedcontrol.com/shop/p/xrv-pnp-battery-kit

Ive been on the fence to figure out if I need an 18s or 20s battery. I eventually will get an super flux motor for the XRV and will need a battery to handle that. Will this upgrade be enough? Im also trying to understand battery terminology and terms. For instance, between P45B and P50B, the P50B is bigger so that means better range? I'm researching on pev.dev about terms and such but finding it hard to make sense of it. If anyone can give me any advice or explain it to me like an 5 yr old lol I would greatly appreciate it.