If there's a stream of riders getting stuck, then the rider(s) you help then have to help the next person who gets stuck, allowing you to ride off. The basic rule is that you help as many riders who have helped you. But you can help more if you want.
Very occasionally you'll get someone who thinks that other riders are there to help them alone, but even then, most of the time they'll only need a gentle reminder ("once we get you out, you're going to help this bloke, OK?").