'Hundreds and thousands' is a plural-form non-singular noun, not having a generally accepted term for the individual element. If you were skilful enough to drop just one, you'd say 'I've dropped one' unless you were feeling whimsical. There are parallels: with confetti, the word 'confetto' exists, but it's probably hardly ever used for the bit-of-paper sense. The singular form 'an oat' is rare.
Mass nouns usually take singular concord (milk is / sand is ...), but may be etically discrete (sand) or non-discrete (milk).
The problem 'discrete referent treated grammatically as having no singular' is discussed in this article by Anna Wierzbicka.