Usually 70-80, but my last purchase was yesterday. I still need exotics but the last 20 dropped at 334, so, I'm no longer wasting my strange coins on that...
I really don't understand the people saying it's not worth it, that they use 20-30 and don't get any. That's either an exaggeration or an extreme case of bad luck, and if you do run into drought continue to use them as it will balance out. I buy about 60 -70 a week, i always have one equipped, and i use them on all characters. Sometimes i may go about 14 in a row without one but then get a second one quickly. Even if it were 1 exotic per 10 coins its still cheaper then buying an engram/exotic item, and it averages at least 1 per 10. Plus it makes every task a little extra fun because of the added chance at an exotic drop.
I buy as many as I have coins for each week. I usually get an exotic every 4-6 coins used. I don't think its ever been more than 10 at the very most for me.[
As many as I can buy I have 250+ 3 of coins at moment use them every other match PvP or every other strike until exotic drops wait for 4-5 matches or strikes an do it again sometimes they work well other times it is a drought but still get exotucs every week
When i would buy, i would get around 160, but no longer have a need for them..although i have friends who continually buy them just for the rush of getting a drop..lol