You would have to keep track of how many iterations you went thru and break out of the loop once you reached the amount of fibers you spawned. I.e. essentially what .size.times does.
Is there a reason using that isn’t sufficient for your use case? If you’re spawning a known amount of fibers then thats the easiest way to handle it. Otherwise you have to get into more robust concurrency logic using select. 5 use cases for Crystal's select statement - lbarasti's blog has some good examples of how you can use it. The first example there might be a good solution if you are wanting to have some work done that spawns an arbitrary amount of fibers. Could have it send to a dedicated channel to know when its done and when it should break out of the loop.
Related: https://github.com/crystal-lang/crystal/issues/6468