You can set this up in your Assessment module settings area. There is an option to add lock settings (i.e. lock after x attempts) but you do not have to use it. In this case you would not want the module to lock at all. To set this up go in to your Assessment module from the Admin side, then click the 'Settings' tab and scroll down to the lock settings, uncheck the box or adjust attempt numbers as you like, then Save.