What does "low sodium" mean with respect to soft drinks?

I’m looking for a rule of thumb. To avoid high-sodium soft drinks, I can easily read the labels and see that one is higher than another. But how low is low? Are they all too high? I’d like a number: soft drinks with more than x amount are considered high; with lower than x are considered low. How much is x?

When I try to get this information by Googling, I can find lists of sodium contents and advice that tells me to choose foods and beverages that are lower in sodium, but I am not seeing the rule of thumb. What is the right amount of sodium to consume, and what’s too much?