This next question is relatively easy to answer, as the number of patio ottomans that you purchase should somewhat correspond to the amount of seating that you have.
However, if you want to get more bang for your buck, you should consider a single long ottoman, which will fit well into a narrow or L-shaped space. You can always break it down into two or more ottomans to fit into the same space, but you'll be spending more money and the separate pieces can sometimes impart a cluttered look.
The benefit of multiple patio ottomans will be that you can free up more space if necessary. If you need more space, all you need to do is rearrange or move a patio ottoman or two.