Spies -- how many do you usually build early on?

[Dog of Justice]

I understand that nobody here knows the formulas governing espionage success/failure. But you still build and deploy spies without that knowledge... my question is, what has seemed to work? What rules of thumb have you developed? I saw some mention of a +20 bonus being the threshold for practical espionage success, but how do you decide how many spies to send on offense in that case? I tried building more than 10 spies and unleashing them on a hapless Democratic AI in the early game, but nothing happened for the ~20 turns I cared to continue playing (at which point I had Supercomps and Robo-Miners, so it was too late for me to benefit much from stealing something like Deuterium Fuel Cells or Fusion Drive...), so I suspect that any offensive spy beyond the first one or two must not be a good use of resources. Or was I just unlucky?