Fundamentally, Visio has some limitations. There may be possible solutions/work-arounds, but it is unclear what you're trying to accomplish. So, at face value, using text tool, select the shape number only, and change its justification to top and left. This moves the shape number to desired location. The remaining text retains its horizontal justification, but the vertical becomes top-down. This is the Visio limitation. All text in a shape gets identical vertical justification.
