On a really basic level, when you edit now, you're actually "authoring" the post again — that is, making a new transaction to the chain. If you take a look at your history with something like Steemd, you'll see each time you edit a post it is logged in the chain in a different block. In this way, it doesn't really much matter how many times you edit, as each time you do it is kept in the blockchain ledger permanently.