Venom attacks you gain throughout the story and the count is cumulative, but different than what appears in the "Benchmarks" tab in the main menu. Where the Benchmarks tab shows how many times your Venom attacks connect with an enemy, not all of them will defeat enemies which is key.
Venom Punch and Smash do fairly well with defeating enemies, Mega Venom Blast (unlocked late in the game) is by far the best one that can guarantee an enemy defeat.
As for best locations to use, there appears to be an unlimited enemy spawn if you replay the mission where Prowler saves you from the Roxxon enemies. You can also simply use the attack as much as you can from the time you get it and through to the end of the main campaign and a few missions into your NG+ run depending on how much you used it previously.