How Can I restrict a user to view one page in a project having multiple page?

Go to your mashup list in the composer and click this icon across the mashup page you need to restrict:

Then

Then you may specify permissions on the runtime.

Alternatively, you can get rid of the Everyone organization and set up your permissions group by group for everything, including visibility. That way by default  nobody would have a permission to view your mashups and instead you would set up visibility for each group individually.

If you check your Organizatioins, by default all users are added to Everyone organization, meaning everything is visible to them - this is not a good security practice. Best way is to remove users from that organization and set up all permissions from scratch. Create a separate organization  (1) for your user groups and divide up visibility on different things. Create one organization (2) for all of them for the shared visibility on things.

Add visibility to organization 2 on such things as EntityServices resource, PlatformSubsystem  and AlertManagement subsystems.

This video talks more about the permission levels: https://www.youtube.com/watch?v=HzFqxvgHtpI&index=8&list=PLz1ppcU_kaneagUT9qgQfz3HByf6-9zTF

You may also look at this document on the use of system user The use of System User

You would need to set up permissions on every thing, thing template, service, persistence provider(storage) used in the mashup. You may try collection level permissions - the lock icon next to the collection name (Things, Thing Templates, etc)