。
She didn't have a single pencil.
Literal
She [topic-は] pencil [object-を] one-counter [even-も] did-not-have.
The pattern [number + counter] + も in a negative sentence intensifies it to 'not even one' — so 1本も持っていません = 'doesn't have even one.' The same construction appears in 一度も行ったことがない ('have never been even once') and 誰も来なかった ('nobody came'). 本 is the counter for long thin objects: pencils, bottles, trees, fingers, and (perhaps surprisingly) home runs in baseball, films, and phone calls. The polite past negative ~ませんでした sets a respectful, narrative tone — common when retelling a past situation politely.