Don't worry too much about chests and keys. I have around 40 chests and no keys lol. Keys will drop randomly very rarely and you also have a chance to get them from personal storyline quest completion as well as zone completion. You can also buy them with gems in the cash shop, but they aren't worth it. They have ok stuff in them and the boosts can be useful, but they really aren't needed. Just keep piling the chests in your bank and when you get a key open one. One time I got a key and in the chest I got another key and then got another from the next chest, ended up opening around 10 chests like that lol.