alter table person add cnt int
    constraint CK_CONSS check (pnum < select count(*) from Gender)

CREATE FUNCTION dbo.Count_checkfn()
returns INT
AS
  BEGIN
      RETURN
        (SELECT Count(*) FROM   Gender)
  END

ALTER TABLE person
  ADD CONSTRAINT CK_CONSS CHECK (pnum<dbo.Count_checkfn())

CREATE TABLE Count_table(Count_column INT)
CREATE TABLE Check_table(Check_Column INT)

INSERT Count_table
VALUES (1),(1),(1),(1),(1)

Go
CREATE FUNCTION dbo.Count_checkfn()
returns INT
AS
  BEGIN
      RETURN
        (SELECT Count(*)
         FROM   dbo.Count_table)
  END
Go

ALTER TABLE Check_table
  ADD CONSTRAINT Count_Check CHECK (Check_Column<dbo.Count_checkfn())

INSERT INTO Check_table VALUES (1) -- Works

INSERT INTO Check_table VALUES (6) -- Insert Failed because only 5 records present
                                   -- in Count Table