There's no hard cap. The calculation is:
Code:
Minimum Level Scan-able = Your Base's Level / 2
Therefore at level 9, you'll no longer be able to see level four bases. Then again, this is all purely based on observations. If you do the math you'll see the above conforms to the actual going-ons in game.
3 / 2 = 2 (rounded 1.5)
4 / 2 = 2
5 / 2 = 3 (rounded 2.5)
6 / 2 = 3
7 / 2 = 4 (rounded 3.5)
8 / 2 = 4
9 / 2 = 5 (rounded 4.5)