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The top four ranks don't have a limit on how many wrestlers can hold them.
Yokozuna is a permanent title, usually awarded for an ozeki that wins two consecutive tournament titles, or (especially in case of a very dominating yokozuna already being active) one win and two runner-up in three, but criteria are subjective and decided on a case to case basis. Thus, there is no demotion from yokozuna; the most the sumo federation can do in case of such performance is write you a letter suggesting you to retire and apologise for shamefur dispray.
The title of ozeki is performance-based; an ozeki is expected to win at least 10 bouts every tournament; promotion is conferred upon reaching this level (usually 32-33 wins in 3 tournaments are needed to confirm form); and demotion comes after having a losing score (so 7-8 or worse) in two consecutive tournaments. (I mentioned before that Takakeisho needs 10 wins as a kadoban – this is incorrect; just a winning score will do, but 10 wins are the requirement to be immediately promoted back to ozeki. This is to allow for ozeki who had a bad performance due to injury to be able to return faster, so yeah, Takakeisho only needs 8 wins, not 10 this time around.) There are always at least two ozeki – but there have been situations where an ozeki was demoted freeing up his slot with no other wrestler ready for promotion. In those situations, one yokozuna is placed in the ozeki slot instead. At no point has it happened that there have been fewer than 2 ozeki and no yokozuna available to fill the vacancy, and nobody knows what happens in that case.
There's no limit to sekiwake and komusubi ranks and there's always at least two wrestlers in those slots – whoever is good enough to be near the top. If there's more than four wrestlers competing considered to be at this level, there will be extra slots created to accommodate them.
Usually there's 8-12 wrestlers in those top 4 ranks – but for example last March, there have been only 7 (with only one ozeki, so Kakuryu filled the other ozeki slot). I don't remember a case of there being more than 12 wrestlers that high up – perhaps at one point where there were 4 yokozuna active? Not in 2017 which is last time that happened, though.
There are no weight classes; so there's pressure to put on as much weight as possible before it weighs you down too much to kill your stamina. They do get weighed at regular health checks, though, so at least the rough data on their current weight is publicly known.
Height – for best performance you'd need to have long arms and short legs. Lower centre of mass is better for balance while longer grip is better for grapples. Top division wrestlers are generally around 185-190cm and 140-160kg, so it would seem that past 190cm average body build has too long legs. There's also not many under 175cm primarily because short wrestlers struggle with grapples, so short wrestlers are either extremely technically skilled (Enho, or Mainoumi during the 90s), or invest fully into pushing and thrusting (Takakeisho).
Body fat percentage? That is generally not known, but various data from health checkups implies most have about 20% body fat. To bulk up even more muscle and go for lower body fat percentage is probably doable – but apart from Tochinoshin few invest in that. Well Hakuho looks relatively lean too. Might change in the future? I think the current high fat is a bit of a fluke – response to Hawaiians and Samoans who dominated during the 90s. And it's easy to get a headstart into sumo as a kid if you're fat.