All foods with an AAFCO nutrition claim have to meet a minimum sodium requirement. This is 0.3% for growth/reproduction (puppy, all life stages) and 0.06% for "adult maintenance" (there is no separate table for seniors, depending on brand it can fall into either category), based on 3,500 kcal/kg on dry matter basis (not "as fed"). At greater than 4,000 kcal/kg the calculation needs to be adjusted for energy density. There is no maximum upper limit defined.
So what you'd need to do is actually find out the average sodium content, not just a guaranteed minimum or maximum. But overall it's more important to look at energy density too, since you'll need to feed more of a lower-calorie food to meet energy needs.
For example for THK Keen fed stand-alone the average is 0.18% on as fed basis, or just over 200 mg per cup dry measure (470 kcal), or 43.5 mg per 100 kcal. For a normally healthy 5 pound dog one cup would supply 60% of the weekly recommended intake. By raw weight, turkey breast supplies 44.1 mg/100 kcal, chicken breast 101.8 mg/100 kcal, and 95% lean ground beef 48.2 mg/100 kcal.