Looks like base firmware is 5.55 as version 6.00 wasn't out yet when that console released. So it all depends on if the console was updated at any point after August 2019 which I think was when 6.72 released.
If you are buying it second hand and it is definitely still original factory sealed (which is just a plain circular piece of tape), then no it won't be higher than 6.72. However if it is second hand opened, get the seller to show you what firmware it has on the system info screen.
So long as it is lower than 9.00 though you can still homebrew and everything, my limited edition was updated to 9.03 before I got it, big sadge.
Unless you believe you could get a permanent hack on 6.72 one day, there's no reason not to just use 9.0. I'm someone that updated from 6.72 to 9.0 on one PS4 and 7.55 to 9.0 on another because I got tired of waiting on backports (although I respect the people who do them) and 9.0 is just better.