There isn't any limit on the number of links you can include on your Link-in-bio page, but your account does have a monthly limit on the total number of links you can create in Bitly. Any links you create specifically for your Link-in-bio page count towards your monthly limit.
You can use any links you've already shortened on your Link-in-bio page. Adding these to your Link-in-bio doesn't affect your general link limits, since you've already shortened the links.
Your monthly limits reset at the start of each month.