Does an expiatory oath (kaffarat yamin) become obligatory for swearing at a guest to eat, or at a host not to bring food, and is one expiation sufficient for all cases?

One must respect an oath sworn by Allah. It is commendable to say "In sha' Allah" (God willing) when taking an oath, for whoever says "In sha' Allah" is not bound by an expiation for the oath. If someone swears to do something and does not do it, or swears that another person should not do something and they do it, then an expiation for the oath is required. The expiation multiplies with the multiplicity of the sworn actions, but it is one if the oaths pertain to a single matter.