How much oatmeal do you use for an oatmeal bath for an elderly adult?

You don't use oatml. The preparation used for "oatmeal" baths is not oatmeal. That would produce a gooey mess. During the milling of oats, a fine flour like powder is created as a byproduct. This milling powder is collected, placed in single use packets, & sold to use as a wetting & anti-itch preparation. It is a good product & many patients enjoy it.