Apparently in the book, the rate of introduction was a bit faster initially.
See The Maze Runner question: One quick question.. on Goodreads that asked.
If there's supposed to only be a new boy once a month and the person that's been the the longest, Alby, has been there two years, how can there be 50-60 boys (as observed by Thomas)??
Shouldn't there be like 24 or less if no ones been there for more than two years (fair enough Alby probably hasnt been there for exactly two years but you get the drift)
Have I gotten something completely wrong here or...?
Where one of the answers state:
At the very start of the maze, 30 boys arrived at the Glade, then once a month over the next 2 years. It explains it a little more as you keep reading.
Also a similar question and answer How many guys are in the glade? also on Goodreads.
So, they actually did more than one a month initially, in the book.
So far as I recall they never explained in the Movie the reason why they only sent up one boy a months.
Possible explanations for sending one a month (in my opinion) would be.
There is only a limited amount of area called the Gardens which got water in which they could grow food. So if they put all of the kids in at once they would be fighting over food rather than co-operating and exploring the maze.
They wanted a stable population size, so they were just adding new kids at the expected rate of attrition.