Since using many extensions (browser addons) in tor can increase your browser fingerprint, is there a way to stop this?
There are extensions that do not send any data to the server side and also don't affect the data processed by the browser. Thus, these are secure to not change your fingerprint. You can see a quick overview over what an extension wants to do when adding it (explanation).
I don't know if it's possible to disable parts of add-ons without touching their source code for example. So, stopping the possibly increased fingerprint is not possible, as far as I know. Of course, disabling the plug-in as a whole disables the increased fingerprint, but that's not what you want.
But you should not add extensions to your TBB. From https://www.torproject.org/download/#warning:
We do not recommend installing additional add-ons or plugins into Tor Browser
Plugins or addons may bypass Tor or compromise your privacy. Tor Browser already comes with HTTPS Everywhere, NoScript, and other patches to protect your privacy and security.
Personally, I have several instances of TBB, each for a specific use case. And there's one that has additional plug-ins. But I do not use this one while I try to be really anonymized/secured.