Just a pinch will do. Its all about what it represents, not quantity.
As for the oil. I use olive oil as a base.
If you want to make an oil with certain herbal properties, first cleanse and consecrate each herb and the oil. Then simply place the dry herb in a small oven safe container (I use oven safe ramekins) and cover the herb with olive oil, then place your oven on the lowest heat setting possible and let it cook for an hour. Take it out carefully (hot oil burns badly!) And when it cools, strain it into a jar or bottle with a funnel that has cheesecloth or a coffee filter inside. Voila! Seal it up and store it in a cool dry place. Of course you should charge the oil after this with prayer or chant.
Alternatively you can place the herbs in the oil, and store sealed in a dark cool place for 1-3 weeks. Then open it up, strain it as I instructed above. And you're done.
The oil may still smell a bit like olives. If you desire for it to smell more like the herb, essential oils may be dropped into the oil. Usually its about 10 drops to one cup oil. Some are more fragrant than others and amount should be adjusted in relation to that.
Always becareful with herbs. I usually choose culinary herbs to use because it is very difficult for any of them to produce a toxic effect if any at all. Oil is a carrier, and like its name, it can "carry" the herb into your skin. Oil absorbs into our skin and so will the herb within it. So it is much different than handling dry herbs that we can merely wash off with soap and water with no worry. As I said, culinary herbs are safe besides....
Hot ones (IE: pepper, cayenne, ginger, garlic) should be avoided when mixing into oils. These can cause burns and irritation if skin comes in contact with it. And if it gets in the eyes can cause serious damage. These are usually used in vinegars which take out most of the heat or they are used strictly in powders.