It depends on a lot of things. I always infuse in Olive oil. Depending on what I want from the infused oil, I use anywhere from 10-100% infused oil. What do you want from the infused oil? If you are going for scent, I would use it 100% of your Olive oil because it will take a lot to get any kind of scent.
Here is how I infuse. I fill my crock pot with my herb and then I pour the oil over and heat it on low for several hours. Then I transfer to mason jars and let sit till I am ready to use, anywhere from a week to months later. I have infused Lavender, yellow dock root, dragon's blood resin, sandalwood, annatto, calendula, comfrey, nettle, paprika, and arnica.
ETA: If you want to infuse several herbs at once, go ahead. It wont hurt it all.