There is no as such limit for maximum number of extensions that you can refer in Visualforce page.
Here is the Salesforce documentation where you will find consideration for Custom Controller/Extensions. : https://www.salesforce.com/us/developer/docs/pages/Content/pages_controller_considerations.htm
Thanks, Pratik
P.S. If this answers you question, please mark it as "Best Answer" so it will help other community members too.
There is no as such limit for maximum number of extensions that you can refer in Visualforce page.
Here is the Salesforce documentation where you will find consideration for Custom Controller/Extensions. : https://www.salesforce.com/us/developer/docs/pages/Content/pages_controller_considerations.htm
Thanks,
Pratik
P.S. If this answers you question, please mark it as "Best Answer" so it will help other community members too.