What is the optimum amount of objectives to use per deck also what is the rule for this regarding organised play?

Optimum is 10. This is due to the make up of the objective sets. You generally have 2-3 units in a set. The larger your deck the more chances you have of drawing no units and thus having a poor turn in terms of your board state. I've run with 11 and 12 just for fun. All the pros use 10. I believe organized play follows the rule book which states you can have as many objective sets as you like, just so long as you can shuffle your deck without assistance. 10 is best. Try to find a good combo of synergy, resources and flexibility.

Make sure to consult the FAQ on this site to make sure you do not build restricted decks.