People always say hand in as many as you can at once, and thats fine if it worked for them but I've never tried to get them and have had loads. Taking a quick, rough count I'd say I've been offered about 25 or more. I'd already done them before the DLC, re-did 4 of them after (upgraded Thorn with Xûr, sod doing that bounty again!) and I've done Invective an extra 3 times and SGA 6 times (got it again today). The rest I just abandoned. Yet a friend of mine has had about 5 in the same time frame, he didn't get any for a long time then read about Vanguard level. He hadn't got his to level 3 and read you needed that. So he levelled up and got his first almost straight away. I'm sure I had my first before I was level 3 but maybe not. All my characters have their Vanguard levels above 3 and all of them get exotic bounties. Maybe something in it.