Yes, the ultras have locations where they have been seen (where you can catch them) and also a number of specimens. Each mission to catch a specific type of ultra, you will be told where and how many. You might as well pick one location and look for all of the specimens there. There's no reason to go to more than one place.
You can only catch one type at a time, and you have to catch all the specimens of that one kind before you can go on to the next one. Pretty sure the first (Nihilego) and last one are only one specimen, though, so yeah... just one.
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You can only catch one type at a time, and you have to catch all the specimens of that one kind before you can go on to the next one. Pretty sure the first (Nihilego) and last one are only one specimen, though, so yeah... just one.