Azam - it depends on so many factors. A symposium and conference may be the same thing. The terms are often used interchangeably. A symposium is supposed to be more 'hands on' i.e. round tables, specialist panels etc. The 'rank' should be specified and be able to be cross-referenced i.e. conference proceedings, doi's etc. Seventeen may be fine - but it depends on the length of symposium, location, how 'niche' the discipline is.
Agreed with Dean Whitehead,there are multiple factors , i personally believe that a symposium ,having a combination of of workshops and conference must have better sores as compare to conference, But it also depends on quality of stuff offered, level of participants and field or scope of topics.