The 9th New Jersey Volunteers was formed in August and September 1861 as a regiment of riflemen. This designation was very important since technology was shifting from smooth bore muskets to rifled weapons. Riflemen were expected to be more accurate and, therefore, more effective (deadly) on the battlefield.
The regiment drew men from around the state rather than recruiting from one county or region. While New Jersey is not one of the larger states, there was no trouble achieving the authorized strength. If your ancestor is in this roster you should request his service or pension records from the National Archives.