1,000 years ago, the world's most populous city had approximately 1,000,000 inhabitants.
Today, Tokyo has almost 30,000,000 in its administrative region (~15million in the City proper).
Apply that growth rate to the future...
That's not entirely accurate. Population growth rates depend on a wide range of factors. In the modern era, industrialisation allowed populations to grow quite quickly, but they do hit an upper limit at which point population growth rates slow to a crawl. Factors include population health, comfort, accessibility (to jobs, infrastructure, etc), and of course, resources. The resource limit is the big one, and the primary reason why overpopulation is actually impossible, because as we approach that limit, the population will actually start to decline naturally, as those without resources simply die off. A balance occurs.
In the future, with greater technological advances leading to medical breakthroughs and improved societal comforts, as well as living on multiple planets, it's highly likely that populations on individual planets won't be more than 1 to 2 billion, depending on how accessible those planets are. If they're not easy to access, those population numbers will probably be higher, due to lack of migration.
It's actually a very complex subject, population growth, and to take our civilisation's recent (in the last 100 years) population boom as an example of a constant population growth rate is actually over-simplifying how population growth actually works. You can get a better idea if you look at the growth rates for individual nations. For example, the US, Australia, UK, Canada, and other advanced western democracies all have very low population growth rates. However, if you look at China, India, and many places in Africa that are industrialising, you'll find much higher rates. Those places in the world experiencing their first
'industrial revolution' are experiencing the highest rates of population growth. Those nations that are post industrial revolution, and in the midst of their information age, are experiencing the lowest rates. There are reasons for this, but it'll take an essay to go through them, so I'll leave it for you to look into.