If we can also count franchises which had only 2 games, my list would be:

Monkey Island
Civilization (only the 4 main games though, no expansions)
Kyrandia
Ace Attorney
Grand Prix (by Geoff Crammond)
Golden Sun
Dungeon Keeper
The Games: Summer Challenge / Winter Challenge (by Accolade)
Pro Pinball

First I thought it would be more, especially in the PC section, but I don't have all games of lots of franchises I like, such as Discworld, Simon the Sorcerer, Broken Sword, The Settlers, Age of Empires, The Patrician, Warcraft, Jagged Alliance and so on.

I have 3 Monkey Island games.. so they can't be two. Wikipedia says they are 4, I had to check before I wrote in my post because I'd never really followed.
Though in a way I only really love the first two and didn't care to anticipate more after the third.

I have all Zelda games even the Game & Watch

all Star Fox games

all Smash Bros games

all Rogue Squadron games

the 2 Battleclash games (BC and MC)

orginal Sonic trilogy and Bonk trilogy on VC

Super Mario Bros. 1-2-2 (jp)-3-New

All of the Resident Evil Games for consoles (RE 1 is the WiiMake though)

Kingdom Hearts

Let's see
-All the Suikoden games that were translated into English (plus Card Stories)
-The four Monkey Island games
-All the main series Prince of Persia games (1,2,3D, the SoT trilogy and the new one)
-All Football Manager games (2005 onwards, since that's the year the series changed from CM)
-The four Kingdom Hearts games
-All the main series FF except 11 for some console (1&2 for GBA, 3 for DS, 4 to 6 for GBA, 7 to 9 for PS, 10 and 12 for PS2)
-The four Advance Wars games
-Every Pro Evo since 3

I think you misunderstood my post, alekth. I know of course that there are 4 Monkey Island games, I own them all (by the way: in my opinion 1 and 2 were and are outstanding, 3 is still excellent, 4 is...well, let's say: better than lots of today's adventures).

I just decided to also include franchises in my list which had only 2 games so far (Golden Sun, Dungeon Keeper, "The Games").

I have every game for the following series (every US release at least)

Advance Wars
Fire Emblem
Professor Layton
Pikmin
Paper Mario

My bad, I get it now (should have before seeing Ace Attorney was there too).

Maybe I should replay 3 first, I think back when I played it I just couldn't get used to the graphics.

Which reminds me, I used to have (probably still do in a box somewhere) the Commander Keen episodes (though not the one for the Gameboy Color). Replayed them one night some years ago, drove my flatmate insane with the music.

Uh, let's see.

Devil May Cry.

I HAVE perviously owned all Mario Kart-games, but I have since lost Super Circuit and maybe the SNES version too...

Smash Bros.

I think that's it.