The available number of construction slots are given random on each colonization. When you colonize the same planet multiple times you will get a random amount of fields every time.
The lowest I have seen is 68 the best is 355. You would have to have someone with access to the random numbers code give you an exact range, but that will give you an idea of what they could be. Generally anything over 300 is considered good
I come back with another question! Does it depends the size of the planet with the number of slots? The smaller the planet the more probability on having less slots, and the larger the planet the more probability on having more slots?