So, there's a lot of research on density vs growth and it suggests with less fish per volume you get faster growth. So, the smaller amount of fish you put in there, the faster they'll grow. This is due to food availability, but also water quality.
If you are doing water changes regularly, or have a drip system of some sort I would say you could probably rear 100 without filtration, but lots of plants depending on the surface area. If you did about 50 of them, they would for sure be safe. With filtration I wouldn't really suggest many more than that, but you'll likely see faster growth. This also depends on the surface area, if you have a long but shallow pond, you can stock more heavily than if you have a deep but narrow pond.