In recipes, the plural of cupful (cup full) is cupfuls, presumably because the same container is used more than once. If you fill two cups with coffee, however, you have two cups full. Because the same-container rule usually applies, the plurals are mouthfuls, armfuls, table-spoonfuls, teaspoonfuls, handfuls, etc. Note, however, that you serve four guests four glasses full of iced tea.
Dictionary of problem words and expressions. Harry Shaw. 1975.