When do plaster cracks get bad enough to have an expert look at them
The important thing about cracks is not necessarily their absolute number or size - though if you've got a particularly long one it can be a sign of problems, but whether the number of cracks is growing and/or the existing cracks are getting longer and/or wider.

A new crack should be monitored over a few days/weeks to see if it gets longer and/or wider and how quickly. If it's changing quickly then call in an expert sooner rather than later.

Who's an "expert" in these cases? Well for less serious cracks a builder should be able to advise you. For larger, wider, deeper cracks I'd go for a surveyor as they'd be more experienced in this area and should look harder for the underlying cause. A decent builder should also recommend a surveyor if they think things are particularly bad.