You can choose a few or as many songs as you like. Many clients just request a few key songs and then leave it to the DJ to choose songs that keep most guests on the dance floor.
As a guide the DJ will usually get through about 16 songs per hour. For weddings in particular the mix between background music and dance music tends to be around 2-3 hours of background music and 2-3 hours of dancing music. For a standard 5 hour event you can assume a maximum of 80 songs will be played. Don’t forget also that your guests will no doubt make requests on the night so this will also reduce the number of songs you need.
Best to choose less songs because if there are too many songs on the request list for the DJ to get through in the remaining time the DJ has to choose songs from your list based on his own experience and this may result in your most liked songs being accidentally overlooked.