Azure Stack offers the ability to add Public IP Pools should the one you provided when the installation took place not be sufficient for your needs going forward.  Typically this will be the case when an operator starts to receive alerts in the Admin portal like this:

OK, so this may be an intermittent warning, happening once every so often.  If so, I suggest there's no need to take any action.  However, if you get an alert warning of 90% utilization across all pools, it's time to take action, and that is to look into adding an extra pool.