you can always claim single if married on the W-4. you'll just have more withholding than needed. It will not affect your ability to file a joint return. if you are still married on 12/31/2019, you'll have the option of either filing a joint return or each of you filing as married filing separately. In most situations a joint return will result in the lowest taxes, or put another way, getting the biggest refund or owing the least amount of taxes.
If you want to see what 2019 might look like you can used the estimated 2019 tax section in the 2018 software or use various free websites
here's an IRS website for estimating withholding requirements
https://www.irs.gov/individuals/tax-withholding-estimator
