I can't guarantee a precise figure but looking at the records of the British Library and the London Transport Museum, together with the magazines offered for sale on Abebooks, suggests that it was published quarterly with the first issue appearing in July 1979 and the final one in October 1982, making 14 issues in all. (One bookseller refers to issue 14 as being 'Autumn 1982'. There are no later issues offered for sale and the London Transport Museum's holdings only show '1979 to 1982', so it seems likely that there were just 14 issues).
For a definitive answer try an email to The Transport Trust (which published the magazine through Ian Allen):
[email protected]