After playing this for +- 2 hours I've got a population of 10,000. Each hour I get about 150 extra workers. That means I get 3600 workers a day, and my achievement in just under 3 days from now.
There's two things that affect your growth rate:
- Do you have a premium account or not. (I started with one for free)
- Medicine level (can be increased in university) This helps out a lot. At population of 32,000 and medicine of level 7, I get +244 population every hour.
You also need to be sure your maximum amount of population is higher than 18,000. Check the houses for this.
Besides, you will need to have the quest for this unlocked. I currently have quest for 10,000 population in the 3rd from left quest tab under achievements. Therefore I'm pretty sure the quest for 18,000 will unlock there. Ill update this solution when I know more. Update: It's located there.
Extra note: extra population comes out very handy. You can allocate them to gathering resources, thus earning you more iron, wood and stone. Besides, when they work, they earn taxes, thereby giving you gold.
Thanks to Shadow Kisuragi
for adding information.