There are boards that do well in 12-18 knots (IS 122, Futura 122, and several others boards in the 115-130 liter range.
There are several boards that do well in 8-12 knots.
So, the answer to your question is most likely 2 boards.
One that does well in 8-14 knots and one that does well in 14-20 knots.
All of this depends also on sailor weight, surface conditions (voodoo chop
I am certainly not saying that you cannot sail in 10-20 knots with one board, but
as suggested by the other posters, you are either going to be slogging (not planing)
in the under 13 knots (even with big sails) if your board is on the small side, or you are going to be limiting your performance in 17-20 knots if you have a board that's too large/wide.
There are exceptions (Formula boards being the most notable) but you need to be able to handle a seemingly huge sail (9.0 m2- 11 or 12 m2) to get the full performance from these specialty racing type boards.
If you want to expand this to cover 8 knots to say 30 knots, then probably 3 boards are needed to be comfortably planing throughout this range of windspeeds.
You would also need sails from about 4.0 m2 to 10.0 m2 so probably a minimum of 4 sails to cover 8-30 knots.
It's often better to have a bit of "overlap" in your board/sail/fin quivers as having great conditions, but no gear that really suits those conditions isn't going to be much fun.
The 3 boards/4 rigs for 8-30 knots is the minimum you will need to be reasonably comfortable in all conditions you are likely to find in that wind range.
Of course many of us sail in places that have relatively stable windspeeds and surface conditions, so we can get by with one board that suits those conditions and a couple of sails.
Hope this helps,