In general, you should use as few as possible.

On FireFox browser, the best privacy extension is uBlock Origin. I wouldn't use any other extension for ad blocking or privacy unless it is for something specific you need that uBlock Origin cannot do on its own. NoScript is another privacy extension that is sometimes recommended.

There may also be some benefit to using the extension for your password manager, as it may help you identify phishing links and prevent you from filling in your password on fake or scam sites. I strongly recommend Bitwarden as a password manager.

Installing extensions is unlikely to help you defend against fingerprinting. If you are concerned about fingerprinting, then you should enable resist.fingerprint in the about:config on FireFox, or use either Tor or Mullvad Browser (which you should not install any extensions on).

Unless you're using the TOR Browser or Mullvad Browser, you're already fingerprintable with a high degree of accuracy for those determined enough. If you're that worried about fingerprinting, you should probably be using one of those.

There's no magic number of extensions that would be considered "safest" from a fingerprinting perspective. Any you add will likely adjust your fingerprint in its own way. But as I said, since you're probably uniquely identifiable anyway you can't really get "more unique."

With that said, it's best to keep your extensions to a minimum for other reasons too. Each extension represents an increased attack surface and you have to trust more developers to not be implementing exploitable code directly into your browser. Generally, I find UBlock Origin to be enough and maybe an extension for your password manager or a few other things. I don't generally run more than 5.

It's probably an exponential function with a negative curve. I imagine what extensions you are installing might matter more