Hello, the Oxford Learner's Dictionary identifies this word as both countable and uncountable, giving examples of each form in context: https://www.oxfordlearnersdictionaries.com/definition/english/surplus_1?q=surplus. Another source https://www.wordreference.com/definition/surplus sheds more light on nuances in meaning of each grammatical category: 1. something that remains above what is used or needed: [countable]a surplus of oil. 2. an amount, quantity, etc., greater than needed: [uncountable]the labor surplus in Sweden.

It simply stresses that more than on surplus is being formed, so either more than one commodity, or more than one country, or area, or perhaps if from context, doing so again and again over time, say seasonally?. "The world is now producing a large food surplus." - also possible as you suggest.