You don't need to have 6 characters, that's just the maximum.
I have personalized plates but it's my name, not cat themed. I do love personalized plates .
SPYNTR (idk if people would get that one)
RESCUE/ADOPT (would probably need more cat themed decor so people know who you're talking about rescuing or adopting, lol)
TNRFTW (another one you'd probably have to explain a lot)
FIXEM (best paired with a spay/neuter bumper sticker)