For anyone trying to get the settlement achievement. You can do it with 1 settler.
What you will need;
1 x Settler (moved from other settlement)
1 x Sleeping Bag (or any bed, must be under cover)
1 x Machine-gun Turret (5 Defense)
1 x Bar* (20 Bonus Happiness)
Local Leader Rank 2 (min. level 14) No other perk required.
Note; * Depending on the game localization it can also be called Shop but basically the Food & Drink Rank 2 store.
Basically fill the workshop with purified water and mutfruit that way there is no need for farms and waterpumps. Make sure you fill the settlement (preferably the red rocket) with crates so the Size bar goes to orange.
The process is sleep 12 hours, wait 12 hours and idle in the workshop till the happiness ticks. Rinse and repeat. Also assign the settler to the bar.