All of them, it's still pretty popular today.
http://en.wikipedia.org/wiki/Journey_into_Space
And still gets run on the radio
http://www.bbc.co.uk/programmes/b00clkyv
Some modern episodes have been commissioned over the last few years but they are not as good as the originals.
To be very pedantic, the first season of JiS is missing, bar one episode, but it was remade in a slightly shortened version a few years later, and that version's complete. Well worth hearing, particularly the ultra-creepy Red Planet and the early X-Files-ish episodes of The World in Peril (the last BBC radio programme to out-rate TV).
The quick summary goes:
Journey into Space (A Story of The Future) - 18 episodes, 1953-54, remade in 1957 as Operation Luna - 13 episodes (the remake omits eps 1-4 of the original, and condenses the original eps 12 and 13 into one episode).
The Red Planet - 20 episodes, 1954-55
The World iN Peril - 20 episodes, 1955-56
The Return from Mars - 90 mins, 1982
Space Force (semi-sequel, originally intended as a continuation with the same characters) - 6 episodes, 1982
Space Force II - 6 episodes, 1984
Frozen in Time - 60 mins, 2006-ish
The HOst - 60 mins, 2009-ish
Oh, and ISIRTA is well worth a listen, particularly for Doctor Why and The Thing, and Professor Prune and the Electric Time Trousers!