I'm curious what a good amount of activations are for Marines.
Around the 2-3k army range.

From what I've been reading if you have too few you can get walked on.
Due to the Marines compact and smaller force composition it seems kinda tricky.

Is 7-8 in a 2k army around the standard?

9-10 would be about right in a 3k game; i've seen some (workable) armies squeeze 11-12 in there.

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Have in mind that if you do thunderhawk assault you'll have "compressed activations" before they come in - you'll use up two or three for the initial assault/land action.

Also if you have Terminators teleporting on say turn 2 that's also one less activation.

I normally aim for 10-11 and prepare to deal with being outactivated at the start.

Activation count is a balance between durability and flexibility.  Too many small formations and none of them pack the punch to take on anything large.  Too many large formations and your army can't respond sufficiently.

Opinions on activation counts vary widely.  You'll find that in the UK there is a very strong emphasis on packing in as many as feasible, while in the US there tends to be more armies at tourneys with lower activation counts.  Personally, I think as long as the enemy doesn't have you beaten by more than about 50% on total activation count, it can be overcome with strategy and tactics.

7-8 in 2K is probably a bit high.  You might have some issues with durability if all your formations are small enough to average ~250 points per formation.  You'd probably be very hard pressed to take on something like a 650 point Leman Russ company, for instance.

When it comes to marines, the number of activations is going to depend a lot on your overall style of play but it should be towards the high end.  In general, Marines have to play to the goals, keeping an eye on what it takes to win the game rather than slaughtering as many enemies as possible.  More activations takes advantage of good SM command and control and allows them to play a fluid game.

Regarding force composition, as noted, heavy air assault will cut down on your effective activations because you'll be using 2-3 at a time and you'll likely lose some in the assaults so your total isn't going to bounce back.  That should be okay because your enemy will lose more.  A ground marine force can use lots of fast attack formations of Bikes and Speeders to provide a fast force with lots of activations.  Though it's always necessary to maintain maneuverability with Marines, I think it's possible to have some good-sized SM formations that can hold ground, like a Tactical formation with maxed out Razorbacks and a Hunter, even though it will reduce your activation count some.

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Neal