Welcome to PlaneShift :}
You're only going to use the skill related to your equipped weapon - so if you have trained "Swords" to 20 but are holding a dagger, it's your "Knives & Daggers" skill that will be taken into consideration, and your damage will be minimal if you've never trained that skill before. Pick one your character would use the most, and if you find extra time on your hands or like a challenge, do as many as you'd like. There are no limits except your time and effort when it comes to how many skills you'd like.