I think a "collection" probably starts around 12 keyboards. At 12, you pretty much have every kind of keyboard worth collecting. Next I would say if you wanted to build a "serious" collection, you need about 25 keyboards. 25 is the number of keyboards you will need if want to have say for example, all the different permutations of the same keyboard, like all versions of Topre, and/ or all the MX switches, and a few others like IBM BS and a vintage Cherry or two. Above 25 keyboards, you are building a "legendary" collection. This is a great thing, don't get me wrong, but now you are venturing into extreme territory, and you will need to devote some serious space and money to your collection. I would say the upper bound for a truly legendary collection is probably around 50. Above 50 keyboards, you are hoarding more than collecting, and you should probably sell some of them so that others can enjoy them. But that's my opinion. Maybe your upper limit is 100? I won't tell you that you're wrong.
How can you get to 25 or 50 or 100 keyboards? It's easy. IMO a truly "serious" collection of keyboards would contain at least the following:
IBM Model M BS keyboard, at least one, but probably 2-3 or more.
Cherry Vintage keyboards: one of each at least of G80-1800 and G80-3000, preferably H and S series, and hopefully a G80-5000 as well.
Dolch Cherry Keyboard
A Cherry ML keyboard
Filco's with MX Blues, Browns, Reds and Blacks.
Keyboards with the other MX switches: Greens, Clears, Greys.
At least one Leo, probably more.
Numpads: Cherry, Leo and Filco
Korean Kustom(s)
Poker(s)
Other 60% keyboard(s)
Ergo Dox
At least one keyboard with modified MX switches, like lubed Clears with gold 62g springs.
A Keyboard with buttery smooth Vintage MX Blacks.
At least one keyboard that you made yourself, like LZ GH, etc.
RealForce 87U's: Silent-Variable, 45g, and 55g.
HHKB Pro 2 Type-S and regular.
Alps keyboard, at least 1 clicky and 1 linear, but probably more.
At least one Back-lit LED keyboard (but don't let anybody know you have it).

As you can see, if you really want to do this hobby right, you can very easily get to 25 keyboards, and probably more.
However, with that said, 2 keyboards is an excellent "starter" collection. One Topre and one MX is a great way to begin. 2 keyboards, however, is simply not enough keyboards. I'm sorry, it's just not. 3? better. 5? Probably 5 is the right number for the OP, at this point in his life. But, I think 12 is probably where he will end up.