https://musicbrainz.org/doc/Style/Classical/Language/English says "Opus and number should always be lower-cased and abbreviated: op. and no. There should be a space before the number." This was intended, IIRC, specifically for constructions like "op. 27 no. 3" and "Symphony no. 5", hence the examples - but there's no clear indication of what to do for a situation like "Five Songs: No. 1".