Adding a small amount of baking soda to your tomato product is an effective way to reduce their acidity. Without actually running a TTA (titratable acidity) to measure the amount of acid present it is impossible to predict just how much soda would need to be added. For most of us, when we have excessively acid tomatoes we just want to take the edge off of them, so a lesser amount of soda is probably the best way to go. While on the topic of pH and tomatoes, please remember that it is the acidity of the tomatoes that inhibits microbial growth with long term storage (think canned). A few years ago a young housewife poisoned her entire family when she canned low acid, yellow tomatoes following the same procedure that she normally used when canning regular tomatoes and used it to make a pasta sauce. The microorganism that grew in her canned, low acid tomatoes was clostridium, and the outcome was not good.
One last thing about pH and tomatoes, it is the acidity of the tomatoes that helps to prevent them from scorching during the baking of the pizza. We did some work on this a number of years ago and we found that the addition of 1-ounce of grated Parmesan cheese per gallon of tomato product did a pretty decent job of taking the edge off of the tartness of the tomatoes. You won't see/taste the difference though until you cook the sauce either by itself or on a pizza.
Tom Lehmann/The Dough Doctor