We would need to look at what resources (land, food, medicine) we have now and what we should have in 10, 20, 30 years time. We would also need to take into consideration how long those alive will continue to live (will depend on country and income) before we look at births. I think we are over-populated now, we are taking away the land and resources from other species, forcing them to come into towns and cities to feed.
The predicted figure is up near 10 billion but really, it depends on how these 1o billion people live. If everyone embraced the importance of sharing the resources, then it would not be as serious an issue. Unfortunately, not everyone believes in this concept and some people in the Western world use way more than their fair share. In addition to this, it is predicted that people will live longer so this will place a further drain on vital resources. There is an urgent need to slow down our population expansion.