Google has updated the Appointment Link field in the GMB dashboard to allow for more than one link.
There does not seem to be any restrictions on the amount of appointment links you can add right now.
On desktop, the links currently show up in sequence. Although it looks like they are reversing the order after the first link.
On mobile, it displays like this:
The additional appointment links gives more options for the business owner to list different ways for a client to make an appointment, for example if a restaurant uses various third party apps for reservations.
However, only the root domain is displayed with no way for the listing owner to differentiate the links from each other. If you would like to point potential clients to different appointment forms on your own site for specific services for example, all the links would display exactly the same.
In any case, more options is nice, but if the ability to add more appointment links to the GMB knowledge panel is to be even more useful, Google would need to add some additional parameters for the business owner to specify what the links are for, either by allowing anchor text on the link or a field title of some sort.
You can always keep up to date with any changes to this new feature on this thread at the Local Search Forum.