The more coils found in a Cartomizer/Atomizer, the faster it will vaporize a volume of liquid. If you seek more cloud production, using more coils can be beneficial toward overall cloud production. The overall lifespan of the coil was theorized to be shorter or longer, depending on how many coils were installed into any given atomizer. This theory is 100% subjective to the device used and user experience level.
One Tank/Atomizer/Clearomizer may perform optimally using a single coil design, while the next product might perform optimally using an 8 coil design... This is something we recommend you experiment with, to best suit your requirements.
Another thing to consider when deciding how many coils to utilize is "hot spotting". A hot spot can happen in any coil, if manufactured with a minor imperfection. A hot spot is essentially a small portion of the coil material that'll heat up much faster than the rest of the coil(s). When vaporizing Eliquid, a hot spot will produce a burnt, dry, unpleasant taste. The odds of experiencing a hot spot will rise with each additional coil installed in any given atomizer. Troubleshooting and fixing hot spots is much easier on rebuild-able devices, opposed to pre-assembled coil heads.