It's possible to guesstimate the number of models that would fit in that space using geometry.
The area of the space within 2" of the hatch is part of a circle (actually an oblong, but I don't feel like doing dual foci this early in the morning). If it were a complete circle, the area would be about 39 square inches.
If you guesstimate that the wave serpent itself takes out about 33% of that area, you're left with around 26 square inches in which models can deploy.
A banshee base is a bit less than 1" diameter, and it's round, so you can something more than 26 models into a 26 square inch area.
And really this is just counting the area mostly entirely within 2" of the hatch. If only the edge of the base is within 2", that could add as many as 8 more models around that circumference.
...and this is assuming that the space is a perfect circle. Actually it's an oblong with the two centers at the corners of the hatches, which makes the area even larger.
If daemonettes occupied half of the deployment area--and counting the 1" rule--there could still be enough room for 12-14 banshees to deploy. It would depend on exact positioning, but it could work.
There's also a dead space under wave serpents/falcons where the model's base is kind of tucked under the body of the model, but clear of its flying base. The rules don't specify whether or not that space can be used for disembarking.
The rule,
btw, says deploy first then move. Usually I add 2" to my deployment distance when there's no doubt about it working to speed up the game. But actually deploying gives you a move bonus closer to 3", so if I think I'm going to need the extra inch, I go through the pain of deploying. Usually I use the bases themselves (since they're just under 1"
to measure the distance from the hatch, just stacking them 3 deep.
...but really, the smart thing for your opponent to do would have been to use a falcon or something to tank-shock the daemonettes away from his hatch first, then deploy the banshees in the space created.