。
Supplies will soon run short.
Literal
Supply [topic-は] soon be-insufficient probably (polite).
まもなく is a formal/written adverb for 'soon, shortly, before long' — more literary than もうすぐ, which is the everyday equivalent. 不足する ('to be insufficient, run short') is a noun-する verb. でしょう is the polite conjectural. The whole sentence has a news-report or announcement feel.