I may have some numbers wrong but the bottom line is interesting.
At release there were 25 maps in PVE.
By season 2 we had only added one more, southsun.
S2 brought us 2 more.
S3 (and HOT) brought us 10 more, not counting the GHs.
S4 (and POF) is 6 so far not counting the GH.
If S4 has the same number of chapters as S3 and is still 1 map/chapter the ratio from original maps to what was added will be about 50/50. The added maps have typically been a lot smaller until recently so land area is still heavily skewed but it will be interesting when a random map being in core is no longer the assumption.
New to the game? Feel free to give a yell if you need PVE help.