If you believe she will receive her SSN before April 15, 2020 then wait to file your 2019 tax return so that you can file as Married Filing Jointly.
Or you can request a 6 month extension using IRS Form 4868 to file the tax return at any time up through October 15, 2020. This is only an extension to file a tax return, it is not an extension of time to pay any taxes owed. Taxes owed must be paid on or before April 15, 2020 to avoid a failure to pay penalty. There are no penalties for filing after the due date if you have a tax refund or there are no taxes owed.
