Cracks in plaster

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.

It could be settling. I see from the window sill it is on an outside wall. You did not mention how long the cracks have been there, but I would go outside and check if there is any water damage happening at the foundation level, missing siding, etc., to rule out settling from decay. Otherwise those cracks, I would say are normal in many cases for a wood lath and plaster home, which should be the case for your 1929 home. The cracks follow the typical way the wood lath is installed.