as many as your feel it can handle. i personaly change my Golf ball everytime but sometimes it can be used for many rounds. most of the time it gets scratches and drit which can effect the spin and compression though. so i would suggest your use one golf ball per round
There is no limit.
As many as you have in your golf bag.
There isn't really a limit of the amount of golf balls you can have. But if you run out of balls, you can't turn in a score card, and you are disqualified.
If it's a friendly game, it's probably time to go home. Technically the rules of golf would allow you to run to the Pro Shop and buy some more. You might get time penalties for this Or you could borrow one from a playing companion.
A round of golf is simply started when a player hits their tee shot on the first hole.
Horse chestnut is one.