It's honestly too early to tell what kind of numbers we'll be seeing, but my goal has always been "1000+ active NPCs." I would be very happy if that number could be pushed to 10,000, but again, it's too early to tell, as I haven't started mass-scale AI optimizations yet (that won't start until the AI behavior is fairly well locked down). As someone already stated, 100 active ships in battle was my goal in terms of individual battle scenarios
Also: Mantle really isn't as relevant in a situation like LT, where we're going to hit a CPU bottleneck long before the GPU taps out. The difficulty will be scaling up the AI and the physics, not the graphics. Mantle cannot help with that part
EDIT : Keep in mind that 1000 "active" NPCs does not mean there are only 1000 NPCs. It just means that you're only seeing "activity" from 1000 NPCs. Regions will be paged in and out as you move about the universe, so it's still entirely possible for you to meet more than 1000 NPCs (or whatever number ends up being the limit of the NPC working set).
Another thing - this number will be very easy to expose so that people with more powerful PCs can try to push the limit