If you did every mission of the campaign, you'll likely have this by the time you finish it.
If you're like me and you used the "El Pollo Diablo" cheat code to skip all the missions that are unnecessary, or for some reason you just don't have it after finishing the campaign you'll have to grind this out.
The game auto-saves after creating a building, if you see the auto-saving symbol in the upper-right your progress is being saved towards this achievement.
Here's how I grinded it out.
I loaded a mission where I had a substantial amount of cash (I had 300k, but I think anything about 50k would work fine) and made a line of shanties ready to be built. When you get to the 50 building limit start at one end of your line and quick build them one by one. Once they are all quick-built, build another line and repeat.
Once you hit around 300 or so shanties, I recommend reloading the save you started on. This game has terrible freezing problems, and too much of anything on screen can freeze it. (You can see if there's a problem by looking at civilians or cars, if they are staggering, or moving slow, it's time to load up the save again).
After doing all the required missions in the main campaign and the Modern Times Expansion this took me around 45 minutes to mop-up.