Normally what si the airchanges per hor for a commercial kitchen if ther is no hood area mensioned. Can anyone tell what is the percentage of hood area in a kitchen.

Try Loren Cook fan company for statrers to size the hood and the fan to drive it. The hood is sized according to appliances, heat out put, temperature, grease,etc.
Check ASHRAE for air change requirements.

Do not forget the makeup air also. Commercial kitchen fans draw a lot of air which needs to be made up in some fashion. Otherwise your customers will not be able to open the front door. Also depending on what area of the country you live in you will need to heat this air in the winter and perhaps cool it in the summer that is made up.