I don't have a ss# for my spouse who I have Not lived with in over 6 years. I am filing married separately. How can I get his ss#? I'm not even sure where he is living.

Did you file a 2018 tax return? If so, I would expect you to have their SSN on that one.

Why do I need his ss# if I'm filing married separately..

Because the IRS says that when a married couple files separately, if one of you itemizes deductions, then you are both required to itemize deductions - even if the itemized deductions of one of you is zero.

Likewise, if one of you takes the standard deduction, then both of you must take the standard deductions - even if the itemized deductions for one of you would be higher.

Therefore when you file MFS you are required to provide the spouse's SSN so the IRS can confirm that both of you itemized, or both of you took the standard deduction. Without the SSN, you flat out can not e-file. You will have to print, sign and mail your return to the IRS.
But I would expect you to have a prior year tax return that would have both SSNs on it. If so, then this issues "goes away".