Rotring

Why do you need several, and if so, which ones? Are there different sizes?

It's compasses in the singular as well as the plural to distinguish
from

It's one of those cases where usage is varied and sometimes confused. Lots of people say 'compass' to mean either of the objects in the above pictures, introducing an ambiguity to their usage of the word. Some people say 'compasses' as if it was a singular, others use it as a plural. I find 'pair of compasses' the clearest, as it preserves the singular, and 'pairs of compasses' for the plural. Obviously, many people will shorten these.

They seem to have doubled in cost (at least) since I was at the Bartlett and last bought some!

The 'some' indicated that Dammit meant several pairs of compasses.