For 40 birds, banties or standards, that tub will suffice for the first 2 weeks or so. Then things are going to get crowded. It's not just about the birds but you have to have enough feeders/waterers in there so everyone is getting enough, and they eat/drink constantly like little piggies. Also, the waste is going to build up at an incredible rate in such a small area with so many chicks. As my chicks get to be around 3-4 weeks old I like to put small roosts in the brooder for them. That will take up space as well. You will also need something to keep them busy (otherwise they may start to pull each other's feathers or pick at each other) so more space yet needed for a few toys like ping pong balls.
Last year I raised about 20 chicks and had twice the space you have now. They stayed in the house in the brooder full time until they were 4 weeks old. Then since the weather was nice they could spend the days outdoors in a protected run and still came in for the night. But I still had twice the space you have for 1/2 the number of birds you are planning. Even at 4 weeks of age the space you have will be crowded for just 10 birds, but possibly do-able.
Last year I raised about 20 chicks and had twice the space you have now. They stayed in the house in the brooder full time until they were 4 weeks old. Then since the weather was nice they could spend the days outdoors in a protected run and still came in for the night. But I still had twice the space you have for 1/2 the number of birds you are planning. Even at 4 weeks of age the space you have will be crowded for just 10 birds, but possibly do-able.