You can use all fresh tomatoes, all canned, or a combination of both. I change it up all the time depending on how busy I am or if tomatoes are in season or not. This time I only used one can of fire roasted tomatoes. I actually like to add it after I mix everything else in the blender. I use an immersion blender to blend it in a little, but still leave some chunks in it.
6 to 7 peeled Roma tomatoes is equivalent to one 14.5 ounce can of fire roasted tomatoes.
The amount of salt needed changes drastically depending on whether you are using fresh tomatoes are canned. Just keep adding and tasting until all the flavors blend together to your liking.