Taller tanks are better. You can keep as many as you are comfortable with as long as you have the right kind of filteration to support them. Also, you do not want to overcrowd them but that should not be a problem because if they are overcrowded then you probably do not have a large enough filter to keep up with the high waste production. Your best bet for a pair is to get them from a hobbiest that has raised them. Also, if you get eggs you cannot be certain whether or not they will be fertile or not. Cuttlefish eggs have a low viability in captivity.
The way I know of determining the sex, which I believe is the only way??? Is to put two cuttlefish in the same tank, separated by a clear divider, the one that reacts by turning black is probably a male, this does not work 100% of the time though. You make sure only to get one male by buying from a hobbiest .
Cuttlefish are coral safe. They will eat fish, shrimp, and crabs (not hermits). Clams are not bothered but you will want to stay away from stinging LPS corals.
To determine the sex of cuttlefish - Multiple cuttles are generally required to accurately determine the sex of your cuttlefish. It can be done with just one, although it is harder and will require effort and patience. After reaching sexual maturity, male cuttlefish will 'display' a unique stance and pattern to show a female that they would like to mate or when fighting other males. In Sepia Bandensis this stance includes the cuttlefish fanning out its tentacles, turning a dark black/brown color with white spots or lines, and angling its mantle upwards (butt-up). If there are multiple males in the area, the other ones will most likely begin to display as well. They will often circle around each other displaying until one attacks the other. Typically, after the cuttle gets attacked it will either back down or, if it is feeling superior, it will bite back. Females will usually not react to this display or will simply swim away.
Taller tanks provide the cuttlefish more room to move up and down the tank more, and encourage swimming and exploring. Shallow tanks usually have less room to explore (more filled with rock/coral/etc.) and you will probably see the cuttle walking along the sand more than swim around. A long tank is not bad, but the cuttles seem to like the depth of a tall tank and IMO a tall tank encourages more activity.
Here is a picture of a male displaying:
He is not in full display, but it is the only photo I have of it. The cuttle to the left of the male is female. Thales has some good pictures of a group of male cuttles with a female in the background.
Actually, i have to disagree on the tank issue for cuttlefish. Their activity suits a wide and long tank rather than a tall tank... its all relevant to what species and what you can afford and fit in your house but, for example, my officinalis tank was 72" long, 30" wide and only 24" deep. There was also a shelf fitted in to the tank which was 48x15 and was about 6" above the substrate, again to increase surface area.
My bandensis tank was 28x24x 18 tall, so again wider.
Both species spend a lot of time on the bottom exploring that way as opposed to vertical travel.