I second the suggestion of “Go Map!!”. The app has a strange title and is really looking forward to an update (there are some issues with the Bing layer in iOS 7, but if you deactivate the background you should be fine). Nevertheless, it is still very comfortable to use on an iPhone for on-the-go mapping and I have used it a lot for house number mapping.

I have also used OsmPad, which gets the job done if you just want to quickly capture house numbers and “post-process” them with JOSM prior to uploading them to OSM.

To be honest, I would prefer if some more development would go into “Go Map!!” (which unfortunately has stalled and is not available as open source), which already does a very good job, than on creating a new app.