First of al, there's no Abel; it's just a metaphorical story: a myth. And part of one at that.
Secondly, nobody knows what Heaven is like.
Earth is in a finite plane, but Heaven may be in an infinite plane, so it may be that All the people who ever were are in Heaven all the time, and there's no problem, because it is the way it is.
You could also quite reasonably conjecture that souls are in Heaven before they go to Earth, and so they are there, keeping Abel company.
Then there's the matter of the Heavenly court. They could keep Abel busy. And that's not to bring up the fact that God is there too.
But the main thing about Heaven is that it's supposed -- no matter what we can imagine -- it's supposed to be better.
Of course, nobody has any clue what Heaven is like, aside from the fact that it's supposed to be a great place.
Hope this helps.
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