Welcome to Librarium Online!
I have created a table for the chance you will fire all your seekers using a certain number of markerlights.
Just go to the number of markerlights you will probably use for seekers in a game, then look across until you find a satisfactory percentage chance of firing all the seekers in a game, then look up to see how many seekers you should equip.
Why? Because I never realised how few seekers I actually needed in a game. Given that I'd probably fire off 20 markerlights, I should only take 6 or 7 seekers to make sure I use them all.