what source of humour are to be found in this scene?

In Act I/ Scene III, we have Polonius and his family as a source of humor amidst the misery. Act One features two appearances by the family. Nearly every Elizabethan play has at least one so-called “subplot,” and this family occupies the primary subplot of Hamlet – the question of Hamlet’s relationship with Ophelia. Polonius, you might have noticed already, is long-winded, pedantic, and meddlesome, even while he is somewhat loveable in his fussy way. He is always interested in being “in the know,” whatever the occasion. Notice, for instance, how eagerly he questions Ophelia about her earlier conversation with Laertes.