I have a 20g and started off with about 500 dubia back in September. They have been breeding so I'm not sure how many I have. If you start off with more then they will likely be ready more quickly. You can probably keep a fair amount in a 10g if you stack the egg crates well. I'd maybe start with 500 or so and see how they go. If it gets overcrowded you can split the colony later on.