If you have a X58 motherboard, you will(usually) have 6 ram slots.
If you populate 3 or 6 of them you will operate in 3 channel mode.
If you have anything else, read your motherboard manual to tell where to put the sticks and what mode they will be in.
Similarly, if you have 4 slots, two or 4 sticks will get you dual channel mode. 1 or 3 sticks mode will be determined by the motherboard, probably all single channel.
Still, the benefit of adding an odd stick for more ram will outweigh any minor loss of not running with the maximum mode.
Ram is sold in kits for good reasons. Ram with the same part number from the same vendor can still be built with different manufacturing specs. Some motherboards are very sensitive to this. It is best to get a kit with the amount of ram that you need.
Most of the time you can get mismatched ram to work. It will run at the lowest common denominator of specs. The key is that all the ram must use the same voltage.