My wife had a stroke a few days after delivering our first child. What's the chance of her having another stroke if pregnant again?

Depends. That is only calculated based on the reason for the first stroke your wife had. There is always a risk of another stroke if the reason was pregnancy related or there is an underlying condition exacerbated in pregnancy.
Depends on cause. That would depend on the cause of the first stroke, if it was found. If you were never informed of the cause, it would be reasonable to have your wife see a stroke neurologist for evaluation prior to her getting pregnant. There are a number of causes for stroke immediately post partum, some are coagulation related and may need to be re-tested. Bring her records from the stroke eval if you can.
Can't tell. Need to find out what caused stroke. Then get % from obstetrician and neurologist.