Journal volume numbering practices are not universal. While a significant number may follow an annual volume model consisting of of some number of issues, that is far from accepted standard. There are several common practices.
- volumes begin with number 1 and contain some number of issues - often monthly or weekly.
- volumes begin with the year of publication and correspond to the year of publication, and consist of issues as above
- the volume numbers themselves are assigned for each issue, which are published monthly or weekly, with no subissues (this is likely the situation you are describing)
There are other possibilities, of course, but these are the most common.
If you consider that a monthly "volume" for a journal would rack up about around 600 volumes between over 50 years, and weekly "volumes" would rack up some 2,600 volumes over the same period, the number you cite is not unreasonable. Just look to the journal's publication history and dates, as well as their "About the journal" page to see their typical publication schedule. As someone above mentioned, this journal publishes every two weeks currently, so assuming that same pace for their entire publication history, they would have published more than 1,768 volumes since they launched in 1956 ((2000-1956)*26 issues per year).