I guess it’s just the ambiguity of the amenity=toilets tag since I’ve understand it to be more about mapping the room where the toilets are located and really about the actual toilets themselves. I think that’s reflected in how the tag is used to. Like you can have an outdoor restroom facility say a national park tagged with building=* + amenity=toilets all day (and before you bring up building=toilets, there’s like 200,000 uses of the former and only like 21,728 of the later). No one maps the toilets as a separate entity from the washroom or entrance way.
So sure, if you added capacity=6 in an instance like that it could very well mean the exact number of physical toilets. It could also mean the number of sinks, how many people total could fit in the room (which is something they have signs for. I saw a few myself when COVID was going on), or maybe something else entirely. It’s not inherent to the thing that capacity means the exact amount of toilets though. Like if it was a bathroom facility with an occupancy sign it would be pretty responsible for most people to assume that’s what the tag is referring to. Not to mention if a tag is being used on a building it’s generally reasonable to assume the tag applies to the whole building, not specific areas or parts of it. At least in absence of any specific evidence to the contrary.