MSSQL - Create table that allows to have only 1 (one) row

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The following table will not allow user to enter more than 1 record

CREATE Table SomeTable(
    Locking Char(1) not null,
    SomeID int,
    constraint PK_SomeTable Primary Key (Locking),
    constraint CK_SomeTable_Locked Check (Locking='X')
)
Insert into SomeTable (Locking, SomeID) Values ('X', 1)
Select * From SomeTable