In IPv4 you're going to need RFC1918 addresses, and then you're going to have to make sure that _your_ RFC1918 addresses don't conflict with any _other_ RFC1918 addresses that inevitably absolutely everything else is using or else you'll get hard-to-debug confusion. No need in IPv6, you should use globally unique addresses everywhere, there are plenty and you will not run out.
Everybody who has ever used a single byte to store a value they were convinced wouldn't need to be more than a few dozen, and then it blew up because somebody figured 300 ought to fit and it doesn't already knows in their heart that they shouldn't be using IPv4 in 2019.
