The answer to your question is not as simple as you might think.
First, IF the mated pair spawned in that tank, with 2 other discus around, it's not likely any eggs or free-swimming fry (if they got that far) would survive. The parents or the other two could and may well eat the eggs (or the fry).
If the pair had not spawned before, being new parents it could take them several more 'tries' before they 'got it right', and fry actually hatched. And if that did happen, the other two discus would need to be taken out of that tank, to give those fry a chance to start attaching to the parents, so as to get nourishment from the parents' slime coatings. Then it would take another week or two until the fry needed to be moved to their own grow out tank, in which one would need to do large, daily water changes, and provide feedings 5 to 6 times a day, if they were to grow out properly, remain healthy, and not become stunted.
It would likely take another month for those youngsters to get to about 1" size, which is about the smallest size that any experienced discus keeper would suggest to a novice discus-keeper to try growing them out to a size where their immune system is strong enough to deal with any problem conditions with changing water params, etc. Growing out young discus is quite time-consuming, and requires experienced and careful attention.
However, re: your question:
Let's assume the unlikely event that there was a spawn that was successful, i.e. some fry became free-swimming, got nourishment from the parents, and you were then able to move them to your other 40 gal. (either that, or remove the parents from the spawning tank).
This is assuming too that you had done your homework, and were prepared to try to grow them out properly in order to save them.
You could actually keep quite a large number of youngsters under 1" in size in a 40, at least for a while.
From 1" to say, just under 3", you could keep approx. 20 to maybe 40 in that tank, following which the remaining near 3" juvie discus would need to be moved to a larger tank.
I know this isn't an answer you were looking for, but it's basically the realities of the matter.