Hi there, interesting question. It's funny how such a small change in the sentence can change the meaning so much! :) 'How many can there be?' is asking how many can exist in total. 'How many can be there?' is asking how many can be in one place at one time. So in your example: "How many participants can there be?" -> "There can be a total of 50 participants." "How many participants can be there?" -> "Only 10 participants can be there at one time. The other 40 participants will have to wait until the first participants have finished." Let's take the simple example of a football competition. How many football teams can there be? Well, we have the whole day to finish our competition, so there can be lots of teams with no trouble. Let's say we'll have a maximum of 20 teams. How many teams can be [on the football pitch at one time]? (...can be [there]?) Well, only 2 of course! Football is always played between 2 teams; no more and no less. Hope that helps!

The first indicates the quantity that are allowed to participate. (There can be 20 participants total) The second focuses on how many will actually attend. (10 of the participants will be there [at the park])

Answer below :)