He's speaking plain english.
There are currently 87 maps. There are currently 216627 members. That's 0.0004 maps per member. He's wondering if the number of maps has grown at about the same rate as the number of CC players has grown.
It would probably be easier to keep track of the ratio as players to maps, so there are currently about 2490 CC members per CC map. I too would be interested to see how that number has changed over time... I'm betting the number of players has accelerated, but so has the number of maps... so it might actually be about the same ratio over time.
EDIT:
The reason he said "whether the average map gets the same amount of play as it used to, say, a year ago" -- if there are a lot more maps per player now, then the average map isn't played as often (of course, maps like classic are played more, but obscure maps like discworld are actually played less, because there are so many more maps). However, if there are
less maps per player now, then the average map is played more often.
of course, this assumes that the average player is just about as active as he used to be.