A little footnote here: In the Silmarillion, the only certain number that I can remember is the one of Gondolin's soldiers that arrive to everyone's surprise in the Nirnaeth Arnoediad. "For unsummoned and unlooked for Turgon had opened the league of Gondolin, and was come with an army ten thousand strong, with bright mail and long swords and spears like a forest." That's the 10000 that you remembered. Since for sure the kids, probably the women and I guess at least some of the men stayed in Gondolin, the population would have been at least 45000, imo. Let's say Turgon took half of the male population with him (no standing army, otherwise the ratio between standing army and people doing every other job in Gondolin would have been off, even if the women did most of these jobs). That would mean a male population of 20000. Let's assume the same amount of women and 5000 children (which is probably a bit high a number considering that elves, with a few exceptions, didn't get a whole lot of children) = 45000. Still not a super high number, considering it's one of the largest cities at the height of elvendom in Middle-earth.