Select the number. In the appearance panel, add a stroke and drag it behind 'Characters'. Give the stroke an opacity of 0%.
Group the number and the rest of your composition. In the Transparency panel, tick 'Knockout Group'.
This will give you the visual appearance. If this is a logo file it may be desirable to actually delete the space created by the stroke, in which case you may need to expand appearance and use pathfinder or shape builder to delete the stroke as well as the area it overlaps.