How to add multiple columns to a table and add default constraint on one of them?

You can use this:

ALTER TABLE dbo.MamConfiguration
ADD [IsLimitedByNumOfUsers] [BIT] NOT NULL DEFAULT 0,   
    [NumOfUsersLimit] [INT] NULL
GO

or this:

ALTER TABLE dbo.MamConfiguration
ADD [IsLimitedByNumOfUsers] [BIT] NOT NULL 
        CONSTRAINT IsLimitedByNumOfUsers_Default DEFAULT 0,
    [NumOfUsersLimit] [INT] NULL
go

More: ALTER TABLE

Try this.

ALTER TABLE dbo.MamConfiguration  
ADD [IsLimitedByNumOfUsers] [bit]  NOT NULL DEFAULT 0,     
[NumOfUsersLimit] [int] NULL