There can be only one Buddha for a group of 10,000 cakkavāla within a given period of time. The next Buddha for us will not appear for a few more million years.
– There can be multiple Buddhas in different groups of 10,000 cakkavāla simultaneously.
I think there is some information on the system of 10,000 cakkavāla in the post I referred to in the previous comment.