They seem to.stress and fret if they don't have enough space each so from that point of view, just to raise one large spawn clump fully you need about a cubic metre. A shallow pond is probably better. That's quite good for tadpoles as long as it doesn't really overheat. If it overheats you have to do something about it. Rig up a temporary shade in summer or have a tree or branch as long as it doesn't drop too many leaves into it.
Some leaves are fine.
Tadpoles like chewing through the stuff but too much of a goodness also makes the water go bad and you get an algal bloom. Let's go over a bit closer and have a look. It's got a plastic liner and also the plastic liner is shade cloth pushed against the plastic liner and planter pots stand on top of it. The purpose of the shade cloth is firstly the fish I have in the top pond then can't go over the waterfall or baby fish even and to the bottom pond where the tadpoles are. And secondly, if a possum or a puppy falls in it can scramble out again because it's not a slippery surface.