It depends on the characteristics of the clay and other materials you have. Natural clay can be a component of soil in amounts from 0 to nearly 100%; many subsoils (not topsoil) have 10% to 25% clay and make good cob, perhaps with additives.
This quote from
Green Homebuilding is appropriate:
"The reason there is no standard recipe in The Hand-Sculpted House is that there is no standard recipe."
All cob has to be mixed to use the materials on hand, testing as you go to find the best mixture. You want to end up with something like 20% clay, 80% sand (and possibly gravel), and enough straw to give tensile strength to the cob - the page mentioned says up to 10% straw by volume. If your clay soil already has a lot of sand and gravel, you might not need to add much.
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