It may be quite difficult to persuade people to come to a launch / event if you have just one short story in an anthology with around twenty other people. You may be able to do this if you have a back list but it can still be tricky.
Here are a couple of suggestions as to what you may try.
Combine with other writers in your area
Find out who lives locally to you and combine with them to organize a physical launch. The whole is greater than the parts. You can pool ideas, share work and introduce your readers to new authors. This is actually a win / win situation.
Work with other authors online
Get together with several authors from your anthology or with several authors whose material is similar to your own. This would be conducted in very much the same way as the on-line launch described earlier though can be very effective because again you are accessing each other's readers.
Here's one I prepared earlier:
Food and drink
Information about authorsNotice too how each writer had a time-slot. This makes the event easy to manage and keeps the pressure off individuals