There is no simple answer to this, as the statistics vary along the line of the Wall. Moreover, it seems there is more than one way of counting a visitor! However, it has long been well known that the central sector, and especially Housesteads, attracts the largest numbers; that site enjoyed a peak of over 150,000 annually in the 1970s but has since declined to around 100,000.
Since the opening of the National Trail, a new type of visitor, one not necessarily overly concerned with the Roman Wall, but rather with completing the Trail, has appeared. Automatic counters set up along the route show more than 7,000 people a year walking the Wall. In the central sector this climbs to well over 30,000 at Steel Rigg, reflecting day trippers out for a stroll (intriguingly classified as ‘amblers, ramblers, and scramblers’).
Further reading: Ewin 2000