{{#imslpmsg:addfile}} Use a pipe after addfile to separate, then put the number by which you want to increment the file number displayed; for example, {{Addfile|1}} would increase the file number displayed by 1 (i.e. one cross-referenced file in addition to the files that are there).
{{#imslpworkentry: Concerto for Piano and Orchestra No 1 in C minor, Op.33 | |Concerto |Instrumental (other) |Op. 33 | |3 | | |Early 20th century |1914-1918 |Piano and orchestra |1918 }}