With the 7-billion-people-day coming up on Monday, UNFPA has a nice chart where you can play through all the scenarios in the next 90 years.
http://www.7billionactions.org/dataJust go to "by country" first and then change the orange button on the bottom of the chart to 2100.
You can also change the fertility between medium, low, high and constant.
Pretty interesting for example Nigeria:
2100 with low fertility: 505 Mio. people
2100 with medium fertility: 730 Mio. people
2100 with high fertility: 1025 Mio. people
2100 with constant fertility: 2661 Mio. people