Wait so they've never established how many moons Oerth has? If so thats a change... not addition... an addition would be fleshing out said moons in accordance with already established lore.
It has two according to the TSR1015 boxed set, page 4. I think it is mentioned in the original folio as well. Luna and Celene are the moons. Luna corresponds to our moon. Celene appears smaller and a aquamarine colour.
*edit*
The same text does appear in the original folio on page 4.
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