In Wings of Liberty, the maximum number of units (based on the supply cap) was 200, but I found out that you can have up to 255 if they are added or found during gameplay. What is the maximum number of units for Heart of the Swarm? I am still in the download process so I cannot check myself.
One zergling take 0,5 supply (and this is the cheapest zerg unit) so you should have maximum number of 400 zerglings on map.
After my small 45+ Blizzard minutes research I gave up.
I played custom with very easy Terran bot. I focus on building Overlords.
As you can see; 342 (unit produced) - 21 (unit lost) = minimum 321 units alive in the same moment . I don't receive any waring or other information that I can't produce more Overlords. So... you can have probably max map number of Overlords.