I have fully researched all the tech areas. I have level 3 in fertility but I am unable to make anymore babies and my population is capped at 90. How do I increase it? Currently, I have built 3 more huts making the number of huts a total of 5 plus a school. Please help.

90 is the maximum population. You can how ever get up to 92 if your mother has trip's. But other then that 90 is the max.

I have seen the screenshots and saw the population number to be 400 to 500. Is that possible?

The maximum population is approximately 90 villagers. Those screen shots were done in a development version of the game, and there's an explanation here.