I think I understand mining well enough, but what I don't get is how the blocks of transactions are assembled. According to this post - What exactly is Mining? - the miners assemble the blocks and then solve for the difficulty.

So, once a block is solved and the miners need to create a new block, how do the miners decide how many transactions to put in the block? Could the miner just assemble a block with a single transaction? Could certain blocks contain more or less transactions than another? Thanks!