You are correct in saying that each bar must represent a range of numbers. Unfortunately it is not publicly known how many each bar represents, or even how it works. For example, according to the official page, the population cap for the Cyrodiil campaign originally launched at approximately 2000 players, but was then substantially lowered by some unknown amount later. According to Brian Wheeler, the lead PVP designer:
To give as much balance as we can per side we do have population caps per Alliance within Cyrodiil.
For the Vivec campaign, as far as I can tell there has never been any maximum server population data that has been publicly released. It is therefore not possible to accurately determine the range of players each "bar" represents on the population screen apart from a rough estimate. The only information available is from the experiences of other players themselves who estimate that the population cap for any given campaign is between 200 to 300.
It is therefore reasonable to assume (given the available information) that if the population chart has only one bar, the server population for that faction will have a lower limit of 0 and a maximum limit of 66 to 100 players. More than that would be two bars up to 132 or 200 players, and more than that threshold would be three bars. This is an extremely rough estimate based on nothing more than subjective reporting, but will perhaps be a good rule of thumb.
It is worth noting as well that server populations are not dynamically capped, which would mean that the maximum server population changes in accordance to the current team balance. Brian Wheeler also says this in his interview:
Currently we have a cap per Alliance within Cyrodiil Campaigns but it does not dynamically scale