Some blends use a limited number of malts, around a dozen, others, many more. 

It is said that you cannot make a good blend with only one grain whisky typically two or three are used.

Blending whisky was first done in the 1820s by grocers who sold tea, coffee, wine and spirits. It began as a way of making the harsh, fiery and often rather nasty whiskies of the day more palatable and developed into a technique for creating drinks that had broader appeal and that were consistent from batch to batch.

91% of the Scotch whisky made goes into blends - 30 years ago it was 98% - and without this marvellous beverage of universal appeal there would be no Scotch whisky industry.