Yes, "biurko" is singular, "biurka" is plural, but "biurka" is also the genitive form of singular (see https://en.wiktionary.org/wiki/biurko). The verb "potrzebować" is used with the genitive, hence "To jest biurko. Potrzebujemy biurka." = "This is a desk. We need a desk". In plural: "To są biurka. Potrzebujemy biurek" = "These are desks. We need desks".
Yes I wish they'd include this with the notes or something! I can look up how to use the different tenses somewhere else but there's so many different situations where you use one tense or another it's hard to find a good explanation on when to use them.
Well, until then... all hail mowicpopolsku.com haha