I've said all the way through this that you don't "have to have" 3 boards and 4 rigs!
If you select your boards carefully, and if you sail in one area most of the time, you can easily get by with one or 2 boards and 2 or 3 rigs.
Around 12 knots is the real decision point.
If you want to sail in 12-18 knots, one board and maybe 2 sails will do nicely.
If you want to plane in < 12 knots, that takes specialized and expensive larger gear.
You can choose to not sail in < 12 knots and then you don't need the larger gear.
Same at about 20 knots.
A 100-120 liter board (depends a bit on the sailor's weight) and a 7.5/6.0 m2 sail quiver
and you can have a good time. Might be a little over powered and the board may be a little big at 20 knots, but you can do it.
The posters here have been asking hypothetical questions like "how much gear does it take to cover 10-30 knots".
I've been providing real world answers.
You could go out with a board and sail that is designed for 10 knots of wind.
In 20 knots you might be able to sail sheeted out, and bounding over the chop (I've experienced this many times when the wind speed jumped up) but it would not be fun.
I took a ride in the St. Francis Yacht club RIB safety boat a few years back at Crissy Field.
I was on an appropriately sized rig, but my friend was on gear that was too big when the wind suddenly increased from around 15 knots to well over 20 knots.
I took the big gear and gave my smaller gear to the friend so they could sail back in comfortably.
I spent a good 15-20 min. trying to waterstart and sail back in but the gear and the conditions were just too much.
So, the safety boat came by, I rolled up the gear and took the ride.
So, the bottom line here is that at some point, your big gear becomes too big for you sail it, and at some point your small gear becomes too small for you to effectively sail it.
In either case you need gear more appropriate for the conditions to have any fun.
Hope this helps,