That's a reasonable counterargument, but I think it's dangerous. The answer could be hundreds. It could be zero. The only thing we do know is that in order to kill him *at that moment* it costs 7.
When you open things up to that kind of speculation, people can justify all sorts of bad decisions. What if they see a terrorist in a crowd of 50 civilians? They could justify those 50 deaths by saying he would have killed more in the future, though they don't know that.
And again, I stress the *at that moment* part. It's inconvenient for Israel, but they should wait for opportunities to strike that will result in minimal collateral damage.
And to take a step back, it's time for this to end. They've destroyed almost all of Gaza. The danger presented by Hamas is far less than the danger Israel creates for itself in the region by continuing this slaughter.