Here are the basics for combining impedances.
For equal impedances, divide the impedance by the number of impedances. So, as warandhairstyle notes above, two 16 ohm impedances gives 8 ohms total, two 8 ohm impedances gives 4 ohms, three 8's gives 2.67 ohms, four 16's 2 ohms, and so on.
For two mismatched impedances, use the product over the sum. An 8 ohm impedance combined with a 4 ohm impedance is (8*4)\(8+4)=32\12=2.67 ohms.
The general case is the total impedance Z=1\(1\Z1+1\Z2+1\Z3...)