How many children does the greek goddess hera?

A lot. All of the deities trace lineage back to Gaia except: Tartarus, Eros (the original, not the son of Aphrodite and Ares), Erebus, Nyx. There are a few people who are descended from those previously mentioned without anyone connected to Gaia being involved as well.

That said, she has at least 47 named children by various gods, including three without the aid of a father (Uranus, Pontus, and Ourea).