Ace Books have published hundreds of science fiction titles, starting in 1953.
Many of these were Ace Doubles (dos-a-dos format), but they also published many single volumes.
Between 1953 and 1968, the books had a letter-series identifier; after that date they were given five-digit numeric serial numbers.
There were a total of 378 letter-series sf titles (62 S&D, 174 F, 19 M, 78 G, 29 H, 1 N, 1 K, and 14 A series books).
For more information about the history of these titles, see Ace Books, which includes a discussion of the serial numbering conventions used and an explanation of the letter-code system.