While it did not reveal any specific details like base and boost clocks, AMD did say that the Threadripper 3990X will indeed feature 64-cores and 128-threads, as well as have a total of 288MB of cache (32+256MB L2+L3). This means it has a total of eight 8-core CPU chiplets and should have the same 280W TDP.
With the 32-core Threadripper 3970X launching with a $1999 price tag, you can expect the 64-core Threadripper 3990X to be rather steep but AMD was not keen to talk about the price yet. There are also rumors that AMD could be working on a 48-core SKU as well, but so far, such SKU has not been announced by the company.
With 24-core and 32-core 3rd gen Threadripper HEDT CPU SKUs getting nothing but praise in reviews, the 64-core one is getting a lot of attention, and AMD said it is coming next year, with more details most likely coming at the CES 2020 show in January.