Depends what you mean by 'poisonous'.
Thousands upon thousands of snakes of very mildly venomous (but essentially harmless) species are kept as pets (e.g. Hognose snakes, Garter snakes).
A small proportion of reptile keepers own snakes that are moderately venomous, but not significantly dangerous (e.g. Mangrove snakes, False water cobras). Keeping of these often isn't restricted by law.
An even smaller proportion keep dangerously venomous snakes (cobras, vipers etc), usually under licence. Here in the UK, keeping is restricted under the Dangerous Wild Animals Act, which involves annual renewal, and inspection of the premises where they're kept. I've met hundreds of people who keep snakes, but only ever two people licenced under the DWA, both of whom kept small vipers.