Question: Is there a minimum number of nights we have to stay?
Answer: Sometimes. This depends on the bed and breakfast, the season of the year, and/or the time of week that you will be visiting. All of this information should be made clear on the inn's web site.
However, even if a minimum stay is required, exceptions are sometimes made -- especially if you're making a last minute reservation and the inn has a room to fill. So feel free to ask, but don't be upset if an inn cannot or will not make an exception.