Consider almost any song in a church hymn book. Either at the top just above the ledger lines or at the bottom near the credits is a series of numbers usually a set of four to six numbers such as 87 87 87 or 10 10 10 10 10 or 88 88 88. They are not necessarily all the same number. What do those numbers mean? What part of music theory discusses them?
The numbers you are referring to are most likely hymn meters. No, not a scale of how well the hymn was received, or how loud it should be, but actually an indication of the meter of the hymn. (Wikipedia has a good article on hymn meters.) The numbers themselves refer to the meter (number of syllables) of each line.
Thus, 87.87.87 would mean the first line has a phrase of 8 syllables followed by a line of seven syllables. The second and third lines would be the same.
In theory, one could mix and match the words and musics from all hymns with the same numbers. Thus, (using the 87.87.87 example), one could sing the words from "Christ Is Made the Sure Foundation" (Latin/Percell, Methodist hymn #559) to the melody from "Angel from the Realms of Glory" (Montgomery/Smart, Hymn #220).
The 10 10 10 10 example is interesting as the 10 stands for 10 syllables, not 1 syllable followed by a strange non-syllabic utterance. In this case, the meter is 4 lines of 10 syllables each.
BTW, many hymnals actually include an index of Meters (Metrical Index) which lists the meters and gives the hymns that use them. The index also shows the Hymn Tune (note progressions) that are used. (For instance, "Angels from the Realms of Glory" and "Easter People, Raise Your Voices" both use 87.87.87 and the Regent Square.)