Hi,
These are called "Buckets" and there will be as many buckets as your CPU's physical and virtual cores sum.
For example if you have a very recent 6 core CPU of today (which has 6 Physical and 6 Virtual cores), You wil see 12 of these buckets rendering your final image.
There are actually two types of render processes;
Bucket rendering and Progressive rendering. Your question is about Bucket rendering so i wont be going into depths of rendering types.
Hope this answers your question.
Best regards;
