My wife and I have around $900 of charitable deductions entered, but only $300 shows up on our return. Should it be $600?

The maximum amount is $300 on a 2020 tax return when filing as Married Filing Jointly. 

In tax year 2021 the maximum will be $600 when filing as MFJ.

I agree $300 is the correct amount for couples.

This assumes you are taking the Standard Deduction.