There is no simple answer to your question like 2, 4, 8 GB ram would be enough.
You would definitely want to enough RAM in Author instance machine so that it does not affect the productivity
When it comes to author sizing guidelines, a lot of factors needs to be considered like below but not limited to
1. Parallel Users
2. Page Creation, Updation & activation In an Hr
3. Volume of content that is being created in AEM
To a extent it is decided by what exactly you are doing in AEM, normal page content creation would definitely require less memory as compared to a site where 100s of assets being injected in AEM and workflows are executing on them.
Having 2-4 GB is way less to run aem properly for author creating content and it does not give any margin for aem to scale in case it has to process more, might even go OOM, Have a minimum of 8GB RAM for author machine to start with.
Here is one of detailed document on hardware sizing from Adobe