Here's something to think about. Let's assume that, like Wings of Liberty, every mission except the first one and the last three introduces one new unit.
Heart of the Swarm isn't going to be the same as WoL. There may be 3 missions with no new units, but with 20 missions I'd think it's more like 2.
Starcraft 1, BW, WoL, and even Warcraft have a theme. The first mission begins off as a combat mission with a select few units to control. I'm guessing that HotS will follow that theme, and will start you off with Zerglings/Kerrigan. (1)
Then the next mission will introduce Drones and Overlords.(2) Spine Crawlers would fit in here,(3) and after that Banelings will join the fray.(4) Roaches will come next,(5) followed by Hydras.(6) Then, going by the gameplay people got to preview at Blizzcon the next mission will be a solo Larva mission which at the end will introduce Queens.(7)
From here it gets foggy. The Swarm Host would probably come in around this time.(8) Mutalisks might join in the party on the next mission along with Scourge.(9) Nydus Worms would arrive,(10) and as a wild guess the next mission would be a solo mission with Infestors.(11) Now that I'm treading in very deep waters, I'll list them off as I see them: Corruptors,(12) Vipers,(13) Ultralisks,(14) Brood Lords.(15) In the trailer for HotS there was a Terran mini-campaign, which I assume has 1-2 missions, which will fit somewhere in between all these missions. I'll go with 2 missions for now.(16-17)
People have been looking through the beta files and finding stuff like Guardians, Devourers, Queen Classic, and Defilers, all with the title "Primal." I'm guessing you will have to face off with an army of these old fashioned units, and maybe incorporate a couple into the Swarm, most likely the Defiler.(18) This leaves us with 2 less then 20, which adds up quite nicely.
Btw, there are Evo-split minimissions to play. I'm not counting them, but they will probably be kind of fun little things to play.