So, is the POP reward going to be given to one computer client each block, or every one?
Doing it every block is far too resource intensive and not needed.
The exact number of blocks will vary per challenge but will always be in a fixed range.
If just one, how is that one decided?
The rewards are paid to everyone who provides proof they host the specific blocks file.
The total reward for POP is set per block and divided over the number of blocks files that exist (this will grow with time) the amount you get depends on the total number of users who provided proof they host that specific blocks file and the number of blocks in the current challenge range.
All decisions are made in a cryptographic, random method very much like how the next block forger is selected in POS.