The phrase "several dozen" means a number that is between 12 and 24. It's an idiomatic expression used to describe a quantity or amount, without specifying exactly how many there are.
As a child, whenever we went to visit my grandparents, my grandma always had a spread of several dozens of different varieties of homemade goodies. (alidaskitchen.com)
Most of the time, however, there are several hundred amino acids per protein, with several dozen of any particular amino acid. (whfoods.com)
Instead of trying to figure out which companies will perform best, you can use this ETF to invest in several dozen of them. (fool.com)