I usually do informal parties but I am going to try to expand this summer and become more professional and hopefully get some more cash.

So I am adding new songs to my playlist:

Broadway: Love Changes Everything, Music of the Night, A Memory, Small World, Summer Nights

Disney: Wish Upon a Star, Beauty n Beast, Under the Sea, Aladin Theme, Circle of Life (piano solo)

Jazz: Autumn Leaves, Stormy Weather, So What, A Train, Night in Tsuania, Summer Time, Unsqaure Dance

Do these sound good to play in a semi-formal setting?

Also do you know how many songs I should have prepared for an evening of playing?

Sounds like a good start.

Add more standards by the great American composers: Cole Porter, Harold Arlen, Jerome Kern, Richard Rodgers, Gershwin, etc.

Pick up some fakebooks, like the All Standards Real Book, or the Just Jazz Realbook, and start playing these.

Sometimes you get into an ethnic thing. It's good to know Italian songs, Jewish songs, Irish songs, etc. Movie themes work, ie, Theme from the Godfather, If I Were a Rich Man, Danny Boy, Lara's Theme, and so forth.

People make requests at pianobars. You can't anticipate them all, but with a proper library of material, and skill at playing from a leadsheet, you can accomodate them. Sight reading and a rich vocabulary of left hand voicings are essential skills for this. Also, you must know your major/minor/dim scales in essential keys (F, Bb, Eb, Ab, C).

You can never know too many songs, but at minimum you should have 50 or 60 tunes in your repetoire.

When playing a piano bar, remember to smile and the world smiles with you.