Sentence:  [T]he same agreeable and mild flavour can be imparted, as far as human experience has yet gone, by rennet, of which the basis is the coagulated milk in the stomachs of calves and lambs. The number of stomachs prepared must of course depend upon the number of cheeses to be made in a season, but prudence suggests the expedience of always having a <WSD>supernumerary</WSD> number. 
Meaning of the word marked with <WSD> tags:  Beyond the prescribed or standard amount or number; excess, extra.