Sorry for not making that clear at the get-go. The more formal name is spline weight, and basically they are non-skid (hopefully) weights placed along, and holding in place, a spline or batten (long piece of limber wood or plastic) that is positioned to connect the dots defining a curve. The dots are dimensions furnished from the table of offsets, or given as dimensions from a baseline as in S&G layout. Basically the goal is to make a nice long fair curve to suit whatever is specified in the design.

or stepping back in the process further, in the old days BC (before computers) someone with a good eye would generate the lines drawing more or less defining the hull shape initially based on lots of experience and a certain amount of black art, then further refine and define and fair the lines with the aid of splines and ducks, and from that accurate as possible set of drawings (even full size on the floor)the table of offsets would be measured off and written down.

As to how many? The smart alec answer is, as many as you need. 8-) But more seriosly I had five as I mentioned previously. That's marginally enough for working on the curves of a small kayak body plan (the rib lines) with a fairly flexible spline (say 1/8" thick by 1/4" high plexiglas)(or wood for that matter)
For the curve along an eighteen foot S&G keel, five probably wouldn't be enough because to generate a fair (smooth) curve where the spline wouldn't deflect from the pencil pressure along it, and still be stiff enough to fair (shorthand for ignore some points that might not be accurately marked)you might need a spline so stiff that five ducks wouldn't anchor it.You get my drift. I guess it's like saying how many clamps do I need?

The short answer is, my best guess is that ten would be luxury for almost anything under 20 feet. If you get a more radical curve than ten can hold, you can always set down a few buckets of paint or the like.

best regards
Ron