First, when it comes to Offering Bowls, can anything be used as one, like a small glass candle container?

And second, I got herbal offerings for DA spirits along with an amulet, how much of the herbs should I offer? Does it matter how much I offer?

Anything can be used and no, it doesnt matter how much. Give as much as you want or as little as you can handle. What matters is its the amount you wanna give and the amount you can handle.

There is no right or wrong in spirit keeping. Do what feels right to you. When I first started out in 2008 I didn't have much to work with. While I had some supplies as a witch, I was a bit under-prepared in my own eyes for spirit keeping. I used a water bottle cover for an offering bowl at one point. A votive candle holder does wonders for an offering bowl. You can even use a plate or anything else, its purely up to you. If you want to get fancy in the future, that's fine, but work with what you have.

Herbal offerings is up to you as well. You want to offer a little so you can conserve what you have, then do so. If you want to offer a lot, then by all means, do so. If you have pets, do be careful where you leave the herbal offerings as some can be toxic to them. Remember, this is your journey, your path. It's all about your intent. You do what feels right and they will enjoy the offerings that you leave for them.