They are Improvements, not buildings   http://prntscr.com/iptl5w

But are you wanting to buy more for a good reason? They cost 200 Rubies each unless you get a discount

I hope you don't think they will give you extra Dak Corsairs. All they do is allow you to revive extra dead corsairs each day (but your friends can do this for you by visiting your cemetery for free)

If you want more Dark Corsairs then you need to spend Pearls in Witch Doctors Hut. This will give you 70 or more, but only if you have 70 or more friends

http://prntscr.com/iptnv0

I refuse to buy any Corsairs Graves because I believe them to be a massive waste of rubies

I have 126 Dark Corsairs and never use them, but could get 8 more by spending pearls    http://prntscr.com/iptsro

I believe the max number of graves you can have is 22. This gives you a daily limit of 25 DCs you can build per day on your own.