Given a set of transactions, and other parameters required to create a block, how many candidate blocks are possible per second? How many of them are valid?
There is an unlimited number of possible candidate blocks since changing any one detail, either the order in which it is presented within the block or, by one bit difference will make a separate candidate block because the random guess and the hash for that block will be completely different. In practice there is usually only one candidate block in existence at a time, sometimes two. Another name for candidate block is potential tip.
Some people refer to the block that a miner is trying to mine as the candidate block, in which case every miner has their own individual candidate block that they work on until it is either a) mined or, b) replaced because a new tip is received.