Yes, you can use the Extra Relics stratagem for each "faction" you've unlocked Stratagems for.
Eihnlazer wrote:You can techniqually get 7 relics in a 2000 point list.
1 for free with the warlords faction and you can buy extra relics for each of your 3 detachments.
Granted it would cost 9 command points and so probably not worth it, but it is possible.
You can "technically" get far more. The 3 Detachment limit is an Organised Event suggestion, not a matched play rule. I'm actually curious to work out now how many I can fit in.
Edit: Thanks to the new
CP counter functionality in Battlescribe, I've worked out it's 19 Relics. As usual Grey Knights ruin it because taking it up to 21 with them goes over the 2k limit.