The difference between the present perfect and present perfect continuous is subtle. Your hypothesis of the reason for the difference in these sentences would suggest that there would be a problem - a 'wrongness' or 'unnaturalness', or a loss of focus on the number, to combine the present perfect continuous with more specific number. Let's have a look:
Staff have been restricting the number of visitors from 900 to 800 per day to slow down the wear and tear on the tower
Hmm. Nope, nothing wrong with that. So what is the difference?
Well, the classic "textbook" explanation of the perfect is that it refers to completed actions, while the continuous (also known as progressive) refers to actions "in progress" at the indicated time. The perfect continuous generally indicates something that is still "in progress", that has been in progress for some time. That's the textbook answer, as I understand it.
It's okay as far as it goes. However, there are many, many situations in which multiple aspect/tense combinations work. This is one of them. The difference in meaning is purely one of nuance, subtle difference. That does come back to that textbook difference. The progressive emphasises the idea that they are still actively restricting the numbers, while the perfect presents it as an action that has been completed; they have done the restriction, and now the numbers are limited. The meaning is the same, but the emphasis, the nuance, is different.
