Exchange Online Exchange Hybrid Server Limitation

Hi Junaid,

You have to meet the below conditions.

You can request a Hybrid Edition product key if all the following conditions apply to you:

• You have an existing, non-trial, Office 365 subscription.
• You currently do not have a licensed Exchange 2013 or Exchange 2010 SP3 server in your on-premises organization.
• You will not host any on-premises mailboxes on the Exchange 2013 or Exchange 2010 SP3 server on which you apply the Hybrid Edition product key.

You can have may hybrid servers that you need but you cannot have any on-premises mailboxes on it.

You can read more here.

https://support.microsoft.com/en-us/help/2939261/how-to-obtain-an-exchange-hybrid-edition-product-key-for-your-on-premi