If you check a list of the most densely populated nations, a lot of New World countries are near the bottom and almost no Western Hemisphere nation (excluding small islands) are near the top. The Americas was depopulated during the Columbian Exchange and even the Pre-Columbian Americas were sparsely populated in a lot of regions. As a result, the regions of the New World aren't densely populated like many regions of the Old World. There hasn't been enough time and immigration since the colonial era to make many countries there densely populated. And considering declining birth rates throughout the world, North America or South America will probably never be as densely populated as Europe and Asia is.
Let's say though that in another universe, the Native Americans were far more advanced. Even though they had less time to develop their societies and were isolated, by some miracle, the pre-Columbian polities end up achieving technological and sociological parity with Eurasia. By 6th Century AD, the Native Americans have the same technological base and similar political institutions to Europe. The Native Americans also have the same domesticated animals East Asia does due to different fauna migrations. As a result, the Native Americans don't suffer diseases from the Columbian Exchange and they also don't get colonized.
Given these factors what would be the estimated population of North and South America by the 21st Century? North and South America would probably have a far larger population than Europe thanks to their increased size. That said, you can't just do a simple population density calculation because there are many areas of the Americas like the Alaska and the Amazon Rainforest that would be hard to live on. The Native Americans also have access to different crops than the Eurasians do. You do not have to factor in epidemics or wars or natural disasters for your population estimate.