I still see roughly the same amount of shows each year (around 75 - 90) but it seems to me that theatre tickets are being put on sale so far in advance that I am buying many many more well before the actual show.
For example, I bought all the Michael Grandage season over a year in advance of the start of the actual performances. Now I can see why he would want assurances that tickets would sell for what he might have seen as a "risky" venture, but it seems to me that more and more theatres and producers are trying to get us to buy tickets long before the actual shows themselves. Many West End shows are booking into 2014 already and I recently bought tickets for the Priscilla tour in Brighton for January 2014. But, unless my memory is flawed this didn't seem to happen so much just a few years ago.
I know I don't HAVE to buy tickets so far in advance but if I want decent seats for the shows I'd like to see, I feel obliged to go for it, meaning I have the assurance of getting good seats and producers have the assurance of a sold ticket.