A standard full set of hollow and rounds includes 36 planes, 18 pairs. The more common half set is still 9 pairs, 18 planes. If the scale of your work ranges from large cases that define a room down to small pieces like spice boxes that adorn another horizontal surface, then a half set of planes may be warranted. A half set, despite its name, is an extremely comprehensive set and will likely be more than necessary for many users.
9 pairs of planes is not likely the perfect place to start for somebody first considering these tools; 1 pair is also not likely ideal. I always recommend starting your collection of hollows and rounds with 2 pairs.
With two pairs of hollows and rounds you will be able to do by far more than twice as much as you can do with one pair. Not only will you be able to make the same profiles in two different sizes (see left), but you will also be able to mix and match the profiles. With one pair you can make 30+ different profiles. With two pairs you can make well over 100. With two pairs you will recognize the true versatility that these planes allow and encourage.
If you do not know what sizes you want but there is a certain profile you need to execute, send it to me and I’ll tell you what planes are used. Or you can find the radii of the included arcs with a circle template. Otherwise, I often recommend getting a pair of #4s and either 6s or 8s (they cut a radii of 4, 6 and 8/16ths, respectively) if the scale you work to is waist height. Consider 6s and 10s (6/16″ Radius and 10/16″ Radius) if the scale of your furniture is shoulder height. 4s and 6s are a good place to start for piece that will stand upon another surface, i.e. mantle clock.
These sizes are a good size for somebody that makes small to mid range (chest of drawers) furniture and will ultimately be included in the largest highboys, secretaries or case clocks.
You will, of course, need a method for creating rabbets that is both accurate and efficient.
Much more information can be found at my blog, Musings From Big Pink.