What does it mean when a charger supports n cells
Check out how many terminals your charger has. Assuming connecting cells in series, a 2-cell charger should have three leads, usually labelled (-), (+3.6) and (+7.2) or something similar. The (+3.6) lead goes in middle point between the cells and is used for balancing. Li-ion cells cannot self-balance in series, so it's pretty much mandatory.

If you're connecting cells in parallel, a single-cell charged will do just fine, since parallel cells do self-balance by passing current from more charged cells to less charged ones. There are two things to remember when connecting cells in parallel:


Cells must be charged to the same voltage before you connect them together. A voltage difference of 0.1V or less guarantees that the balancing current will stay below 0.5C which is safe for most cells.
Connecting N cells in parallel will result in N times the capacity rating (mA*h), but the C rating will be lower. Unless you use cells with closely matched internal resistance, drawing high current may result in only a few cells with the lowest resistance to be loaded, which may result in a failure.