With a spinning blade you are looking to store as much kinetic or moving energy as possible. In a straight line kinetic energy is calculated by E = 0.5 x mass x velocity squared. When it comes to rotation the equation becomes E = 0.5 x rotational velocity x moment of intertia squared. The moment of inertia is essentially a number that is derived from the location of the mass about the central axis of rotation. The long and short of it is that you get a greater figure when you have more mass further from the central axis of rotation. There are various ways to calculate this. If you don't want to get into detail with it the team cosmos kinetic energy calculator can give you a good approximation http://www.teamcosmos.com/ke/ke.shtml (Have a play with some designs and figures to get a feel for the figures)

For reference featherweights these days can easily store a few kilojoules of kinetic energy and some are hitting 10 plus kilojoules. For reference, hypnodisc had around 3 kilojoules on it's first outing and typhoon 2 had the potential to store 40 kilojoules. Tombstone in the states can in theory go as high as 100 kJ.

You haven't stated what weight category you are considering.

You are correct that less teeth is better but not for the reason that Danny stated. The reason less teeth are important is that you get a greater cutter bite with fewer teeth. Essentially there is a longer time that the robot is moving forward from one tooth being in a location and the next tooth taking it's place as the disc rotates. You therefore end up with more of the tooth able to make contact with the opponent. You need cutter bite to get engagement with the opponent to transfer the energy from the disc. This is a factor of the number of teeth on the disc, the forward velocity of the robot and the rotational velocity of the disc.