This is the number of HTTP requests that were sent to your server in the process of scanning it.
To check for both SQL injection and XSS vulnerabilities, VAddy currently sends at least two requests for each URL parameter. For example, it would send 4 scan (HTTP) requests for http://example.com/check.cgi?foo=aaa&bar=bbb: 2 HTTP requests to test foo and 2 HTTP requests to test bar.