SQL Server 2014 :: Add Multiple Records To Table (insert / Select)?

I am trying to add multiple records to my table (insert/select).

INSERT INTO Users
( User_id ,
Name
)
SELECT ( SELECT MAX(User_id) + 1
FROM Users
) ,
Name

But I get the error:

Violation of PRIMARY KEY constraint 'PK_Users'. Cannot insert duplicate key in object 'dbo.Users'.

But I am using the max User_id + 1, so it can't be duplicate

This would insert about 20 records.

Why the error?