The seed value -- rather than seed + 1 -- is the identity value that will be assigned to the first row inserted into the table.

So, if you want the values to start at 5000001, you must seed that number, NOT 5000000.

You can remember this by remembering that the default seed value is 1, not 0; and you know that idents on new table rows start with 1 (the seed value) rather with 2 (the seed value + 1).

Btw, rows only require a key in theory, not in practice. Thankfully, practical business people build useful, real-life applications rather than solely ranting about theory.