I haven't been able to find anything recording the number of wins but Gamepedia does have some good info here: http://hearthstone.gamepedia.com/Arena#Statistics
For every 8192 Arena players, only 2 will reach 12 wins with 0 losses, making this a 1 in 4096 achievement.
Out of 8192 players 53 will attain 12 wins. Making 12 wins a 1 in 155 chance or about 0.65% chance.
This is compared to the Ladder in which [0.5% of players reach legendary each month].(http://hearthstone.gamepedia.com/Play_mode#Ranking_percentages)
These statistics are still not equivalent however, because the number of players entering the arena each month is unknown. Even if it were known it could be that one player reaches 12 more than once per month, skewing the number of 12-win players.
As far as "arena is getting more difficult", this is a judgement of player quality in the arena which would be difficult to measure. However the skill of individual players will not impact the number of players reaching 12-wins. For every loss someone else won. When two 10-win players face eachother one must make it to 11 wins. When two 11-win players face eachother one must make it to 12 wins.
It is possible that the wins have become more concentrated? For example if a single player reaches 12-wins several times in a month that lowers the number of 12-win runs available to the general populace.