This just means that you can only include letters and numbers in the field (and no special characters like ! or %). I’m assuming this is for a username you’re creating.
REGISTER or login:
To list all combinations would take forever to type in. If you are forced to enter a number to meet their requirements you may just have to keep trying until it gets accepted. Examples: JohnSmith1, JohnSmith2, JohnSmith3, JohnSmith4JohnSmith5, JohnSmith6, JohnSmith7, JohnSmith8JohnSmith9, JohnSmith10, JohnSmith11, JohnSmith12JohnSmith13, JohnSmith14, JohnSmith15....
Some sites will may make a suggestion if your name ID has already been used. Like JohnSmith23456, JohnSmith784, JohnSmith95126. It's just the name and a random number that has not been used to create an account.
If you are getting an error creating the account, could you post the error message and if it's a website we would have access to the web address and we can see what the problem may be.