The total number of paperback editions published in 1955 was close to 1,120. This number, by my calculation, includes 300 titles originally published in paperback. Five publishers accounted for more than two-thirds of the titles: Gold Medal (66), Ace (52), Dell First Editions (41), Ballantine (27) and Popular (26). Five other publishers were also in double digits: Avon (20), Graphic (14), Pyramid (14), Harlequin (12) and Lion (11). By contrast, some popular paperback lines relied almost completely on reprints of titles first published in hard cover. Bantam and Signet, for example, published more than 200 titles between them but only five were PBOs.
I was also interested in what sorts of books came out in paperback originals. Dividing the titles into five categories (crime stories, westerns, science fiction, other fiction and non-fiction), I came up with these stats for the big five:
Feel free to interpret these numbers as you will. For background on paperback publishing in the 1950s, you might want to check out this article by Frank L. Schick from (I think) vol. 7 of Library Trends.