I want to populate my settlements faster, is there any purpose in placing multiple recruitment radio beacons in them?
Only happiness increases the chance.
Build bars ( lvl 3 restaurant) to get +40 happiness.
No new settlers will come if you have five settlers that are not assigned to anything.
Personally I didn't notice any difference when I built multiple towers, but I haven't properly tested it
Make sure you have excess food, water and defence (you need 1 food per person, 1 water per person, and 2 defence per person (so as to keep your defence rating equal to the sum of your food and water)) and people should start arriving fairly quickly
Also note that there's a population cap on each settlement, which is equal to 10 plus the number of points you have in the charisma SPECIAL attribute
multiple beacons make no difference to the amount of people who will enter a settlement according to my testing in the ps4 version of fallout 4. only the amount of excess resources and happiness will increase it (given you already have one recruitment beacon)