How much can I gift my son and his wife together without being taxed?

You will not be "taxed" but if your gift exceeds the yearly gift limitation, you have to prepare a special tax form 709 that is not part of an income tax return.

Gifts given to family members, friends or other individuals are not deductible.   Gifts received are not taxable to the person who received the gift, and are not entered on a tax return.

If your gift exceeds the yearly limit ($17,000 per individual)  imposed by the gift tax rules, then you will need to complete a Form 709 gift tax form and send it to the IRS, although it is very unlikely that you will owe any tax. 

TurboTax does not support Form 709.  It is not an income tax form and would not be included as part of an income tax return.

Here is a link to the form:

https://www.irs.gov/pub/irs-pdf/f709.pdf

https://turbotax.intuit.com/tax-tips/estates/the-gift-tax-made-simple/L5tGWVC8N

The annual gift tax exclusion of $17,000 is for 2023. For 2024 it is $18,000.

You will not have to pay any gift tax unless the total of all gifts you have given in your lifetime is more than $12.9 million (for 2023). But you have to file a gift tax return, Form 709, if you make gifts exceeding the annual exclusion to any one person in one year. Gifts exceeding the annual exclusion reduce your lifetime exclusion.