There is no limit, if you check the top list on the dashboard, you can see that the top 10 donors all donated over 45 000 sentences.
I belive that for languages with small datasets a single active contributor can make a big difference because this person assures that you can create a useful dataset and a first model at all. Bu this model will only be useful for this one person of course. You have to work on the generalization later, but it can be a good proof of concept.
So, once your dataset is becoming big enough to really create models, it is much better to focus on diversity. But I also made the experience that having active donors and a permanently growing dataset is very useful for the morale of the other donors and keeps the community running. Even motivated people are quickly leaving the project if there is nothing to validate.
Another possible thing to do instead of recording is expanding the sentence corpus. This especially important for small languages.