I feel that a few factors in deciding on the number of extensions you install on a browser -
the capabilities of the device - speed of processor etc, your internet speed,
how bothered you are by page loading times that are measured in fractions of a second
how useful the add-on is.
I've got more than the suggested minimalist approach, they are predominantly security and privacy related ones for Firefox- UBO, NS, BitWarden. ClearURLs, LocalCDN etc. I can't see any significant slowing of page loading if I add a few more to try out when using a fairly recent, decent speced PC with a reasonable Fibre connection. I've got a few older boxes which do notice additional add-ons.
Remember the adage that the more extensions you have the more unique your browser becomes and therefore you will start to stand out from the crowd to anyone who is monitoring that.