May I drink water before yoga exercises?
If yes how much?
Seriously?
Yes 
What is the basis for this question?
I know that eating is forbidden before exercises…
[QUOTE=JSK;63644]What is the basis for this question?[/QUOTE]
Forbidden?
Eat too much before doing anything, and you’re going to be miserable.
Hydrate yourself 1/2 - 1 hour before class. It is best not to eat 2 - 3 hours before class. Drink lots of water after class.
Yes, not forbidden. Use discernment. Best to exercise on an empty stomach, best not to drink a bunch of water just before as well. You may end up with stomach upset. But hydrate yourself regularly so drinking directly before isn’t an issue.
Every class I have ever been to, there are water bottles beside most people’s mats…
[QUOTE=JSK;63656]Every class I have ever been to, there are water bottles beside most people’s mats…[/QUOTE]
Although this is true for hot yoga classes, many other styles discourage drinking water during classes. In Ashtanga, drinking water is very much discouraged. Luckily, in a free society, you can do whatever floats your boat. 
I’ve done Anusara, Yin, Jivanmukti, Forrest, Jyoti, Yoga-Fit, and Iyengar styles, most people bring water to the studio and take sips during class.
I think flex penguin has it right.
"Hydrate yourself 1/2 - 1 hour before class. It is best not to eat 2 - 3 hours before class. Drink lots of water after class. "
This will help to not have your muscles cramp. Your energy will be better. Just don’t drink too much.
FlexPenguin hit the nail on the head. George Ohsawa recommended only 3 pees a day but he was kind of extreme.