Is there a way to check a post for number of img tags and disallow if there are too many? I would like to make it so there is only one picture allowed per post
Not built-in to my knowledge, but you could add a couple of lines of code to do so yourself by counting the number of instances of the opening '[img....' tag and deciding on what you want to do if there's more than one (for instance, will you send an error message, or just show the first image, etc)
you could just tell your users they can only post one img and then start locking their accounts if they post more. not sure why you would want to introduce a limit anyway.