Absolutely nothing in the rules with regards to how many maneuvers of any given level is allowed. There is absolutely no restriction on the amount of maneuvers known of a given level or of a given discipline aside from those involving initiator level and the prerequisites, both of which are clearly indicated per maneuver.
What I imagine he decided is based on the amount of maneuvers known by the Swordsage progression, beginning with 6 and ending with 25. Every level you receive an additional maneuver known, and since you gain a new level of maneuvers every other level of Swordsage, you'd have a chance to gain 2 maneuvers each level. The problem is his math is bad since you'll hit level 9 maneuvers at 17th level, gain your second at level 18, and still gain 2 maneuvers known at 19th and 20th.
More importantly, he neglects a good portion of the rules on maneuvers which is identical for Crusaders, Swordsages, and Warblades about exchanging an old maneuver for a new one every even level starting with 4th. Clearly says you can choose a maneuver of any level you want to provided you are the proper initiator level.