can i give away multiple keys in one giveaway?

No, one winner one key.

Though you can add more keys for single giveaway just by having each of them on new line.

I want to advice that you set a minimum level for your future GAs because each one that gave one, not free, game is, at least, level 1.
Easy solution to hold the worst rulesbreakers, win resellers, autojoiner user and bots outside (40% of the level 0 accounts have infractions for unactivated or multiple wins + the ammount of used autojoiners are much more as in higher levels).
And besides that it lowers the cases of problems with your winners.

You can check accounts/your winners with:
http://www.sgtools.info/activation
http://www.sgtools.info/multiple-wins

If someone have red marks there request a reroll and put the link(s) into the support ticket.

https://www.steamgifts.com/stats/community/users
Scroll down to the bottom and look at the "per level" info and you see 970k are level 0, only around 165k have a level, 113k of them are level 1. So around 15% of the members from sg gave something and only 4% gave more as 5 games.

Have a nice day

Also as a new user you won't be able to make more than 3 giveaways anyway due to the limit. (Or 1 giveaway with 3 copies, you basically have 3 slots).

After you successfully send few GAs you can ask for few more slots to make GAs faster. But I don't personally give more slots that winner already have. Everyone should rise their slot number in similar pace. So it would take you a bit time to make GAs for all 70 keys. Even with 1h giveaways some winners can still take couple days before they confirm their win.

I believe another exception to the rule is if it is a complete edition with multiple keys in activation. Like "Skyrim Anniversary" where you have both Skyrim and the Anniversary as seperate keys since it is considered DLC.