20 million would be the peacetime corps (comprising Exploration Division (arrowhead), Operational Support Services [aka Starbases Division, (starburst/flower)], Auxiliary Forces [Merchant Service (hoof), Border Watch & Colonial Defense (pikelet/triangle] and Staff Corps (unknown)).
Frankly, given the likely population of a trillion or more for the Federation, I have previously suggested that the total Starfleet during wartime could be as much as 2 billion, though I doubt more than 20,000,000 would be forward deployed on starships or support vessels at any one time, maybe even as little as 1/2 to 2/3 of that.
