How to limit maximum number of rows in a table to just 1

These two constraints would do:

CREATE TABLE dbo.Configuration
( ConfigurationID TINYINT NOT NULL DEFAULT 1,
  -- the rest of the columns
  CONSTRAINT Configuration_PK 
    PRIMARY KEY (ConfigurationID),
  CONSTRAINT Configuration_OnlyOneRow 
    CHECK (ConfigurationID = 1)
) ;

You need both the PRIMARY KEY (or a UNIQUE constraint) so no two rows have the same ID value, and the CHECK constraint so all rows have the same ID value (arbitrarily chosen to 1).
In combination, the two almost opposite constraints restrict the number of rows to either zero or one.

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On a fictional DBMS (no current SQL implementation allows this construction) that allows a primary key consisting of 0 columns, this would be a solution, too:

CREATE TABLE dbo.Configuration
( -- no ConfigurationID needed at all
  -- the rest of the columns
  CONSTRAINT Configuration_PK 
    PRIMARY KEY ()                -- 0 columns!
) ;

You could define the ID as a computed column evaluating to a constant value, and declare that column to be unique:

CREATE TABLE dbo.Configuration
(
  ID AS CAST(1 AS tinyint),  -- or: AS bit
  ...  -- other columns
  CONSTRAINT UQ_Configuration_ID UNIQUE (ID)
);

You can also use trigger..

create trigger LimitTable
on YourTableToLimit
after insert
as
    declare @tableCount int
    select @tableCount = Count(*)
    from YourTableToLimit

    if @tableCount > 50
    begin
        rollback
    end
go