Actually there is no way to really max out the number. With the right placement you can go on forever (or till the server crashes)
The trick is phasegates. Have one where you want to plant the mines, and another by the armory / base. As long as you don't have the mines interfering with the structure limit for the area you can continue to drop them and place them on the other side of the PG...
Edit: Yeah, looking at the picture explains exactly why it got maxed out. When making towers you never want to place the mines near the armory. That will max out the areas structure limit and cause you to not be able to drop more packs of mines.
Oh, and that tower is hardly huge.. Try for the roof of MS on ns_metal
Tower part 1Tower part 2Tower part 3 (below)