How many coins do you need to get all 12 prizes for Lucky Draw?

That is for Joy 777, not lucky draw event

Answer above is correct. Both Lucky Draw and Joy 777 need 50 coins to get to 100%. OP probably mean Lucky Slot, not Lucky Draw.

Lucky Slot, I have only first 4 draws: 3, 6, 10, 15. So at least 34 of those coins with triangle inside.

It isn't fixed, because in lucky draw sometimes you get bonus coins and sometimes you don't.

3, 6, 10, 15, 20 = 54 coins only opens on average 5 slots out of 12. To get to open all 12 slots takes a ton of coins. Only got suckered into paying for it once for ironclaw frags. Nothing lucky about it. Almost always opens the lower slots before opening the higher ones. This is why you don't see a lot of top prize (frags) announcements in this event be others.