Daniel Tewfik has wrangled up the data on the numbers retired by MLB teams, and put together a slew of visualizations. We love this stuff.
The above chart shows the amount of uniform numbers retired by each team, separated by league. (It does not included Jackie Robinson's No. 42, retired across baseball.)
It's no surprise that the Yankees have the most, including the very first, Lou Gehrig's No. 4 in 1939. In a close race for second between some historically successful franchises: the Cardinals, Dodgers, Reds, Giants, and...Astros? (The question becomes: which teams have lots of legendary players, and which are just extra-generous with canonization?)
Not reflected in the chart are the players from pre-uniform-number days. The Tigers honor seven players by displaying their names on Comerica's wall—if they were included, it'd bring their total to 13. (Tied for second with St. Louis, who would get to add pre-uniform-number Rogers Hornsby.)
Tewfik has more graphics on his site, including the most-commonly retired numbers, and the number of retirement ceremonies by year. Somewhat surprisingly, the rate of retirements has remained steady over time, meaning we have a few more centuries before uniforms have to go to triple digits.