How many fleets can i send at one time?
That depends on the level of your Computer Technology. You can have one more fleet than you have Computer Technology.
For instance, I have a Computer Technology of 12. This means I can have 13 fleets active. An active fleet is one that is not at your planet… so Espionage Missions, Deployments, Trading, Attacking, Harvesting (Recycling) or Colonization all count as an active mission.