It depends on the gig, and how long you are playing for. Most bands will play about 12 songs per hour. If it’s a four-hour gig, and accounting for some break time, you will need about 35-40 songs. Most bands have a larger repertoire than what is needed for any single job, since they might not play the same songs every night.
Also, a client might ask for specific songs (like at a wedding) that the band must learn. For bands that regularly rehearse, it’s a good idea to keep adding new songs to the repertoire, if only to keep from getting bored with playing the same songs all the time. Learning new music should be fun!