Change default email app for 100 Windows PC's

The mailto link will launch whatever application is designated for handling e-mail on that device.  So yes, it can open Outlook, but only if Outlook is the default e-mail application.  There's no way to specify in the URI what application you want to use.

As example, in windows 10, in the windows search box on the lower left, type in "default apps".  You will see, "Default Apps" available in the pop out and click there. You should see, "Choose default apps" where email is an option. If you do not see outlook, click whatever is there and select outlook.
https://support.microsoft.com/en-us/help/4028161/windows-10-change-default-programs

but we have over 100+ pc. it is impossible for us

You can set the default protocol handler via GPO.

https://blogs.technet.microsoft.com/mrmlcgn/2013/02/26/windows-8-associate-a-file-type-or-protocol-with-a-specific-app-using-gpo-e-gdefault-mail-client-for-mailto-protocol/

You can set a file association via GPO, but only for those extensions that don't already have an association.  This prevents malware from changing file associations without human intervention.

Paul can u tell me more in detail how to do that?

My link shows a method where the default handler can be changed. Did you look at it?

This TechNet page shows several different ways:
https://blogs.technet.microsoft.com/windowsinternals/2017/10/25/windows-10-how-to-configure-file-associations-for-it-pros/

Another, more conversational take:
https://social.technet.microsoft.com/Forums/en-US/7829c2c3-e6bf-41e2-b1d2-cd2928906571/default-apps-for-all-users?forum=ws2019