You can create 5 other characters you just need to not have more than 20 active characters at a time. Your account will only hold up to 25 characters, 20 active and 5 have to be in the process of being deleted.
When you manually delete a character on your account on Tibia's website by logging onto your account it will take a turnaround time of 60 days to be fully deleted. You can undelete the character within the first 2 months (60 days) if you change your mind. After this time the character is deleted for good and cannot be restored anymore. Let's say you have 20 active characters and you want to create another character. You need to have one pending deletion on your account in order to create another character. You'll be able to get the rewards on the new character, delete it and then have enough time to undelete your other character. Please note accounts can have up to 20 active characters. I tried testing this out to see if there was a limit on how many characters I can have on the account active or in the process of being deleted. I was able to have a max of 25 characters on my account, 6 in the process of being deleted and 19 active on my account. I then undeleted a character to have a maximum of 20 active on my account and 5 in the process of being deleted. However, as stated earlier it does take 60 days for a character to be fully deleted off of your account and within this time you can restore your character you were deleting originally to make room for new ones.
So to answer your question, in theory, yes, you can collect rewards by deleting one of your characters, creating a new character, deleting that character, and then undeleting the character you first deleted to save room. A good tip would be to read up on the event you plan to take advantage of. Also, get organized by knowing what date the event starts and ends. For the Piece of Cake event, for example, you just need to make sure you can do the cake extractor task while Brutus is there or the day your world completed the event as Brutus is there for 7 days only (as long as the regeneration and experience bonus is active). I'd recommend creating the characters getting them to level 8 at least and premium account. I would do the extractor task, collect my reward and send it to one of my characters that I don't plan to delete. Delete the character then get on my 2nd noob and repeat and so on until you've reached the maximum of 25 characters on the account. I've personally never done this myself but I think that this is a good Tibia tip.