... (maximum) can be formed with n straight lines?
The answer is (n choose 3).
That this is an upper bound is clear, as a triangle is defined by 3 lines, and there are only (n choose 3) ways to pick 3 lines out of n lines.
On the other hand, this can be achieved by drawing n non-parallel lines, no three of which are concurrent. This works because any three non-parallel, non-concurrent lines define a triangle.
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