In 8.1 version the SMP can manage 35k agents in the maximum (Page 55).
In addition, in AWS website there are three kinds of ITMS options using xlarge, 2xlarge or 4xlarge machines. I think if you pick the 4xlarge you will be able to manage 35k agents per server without any problem.
I'm attaching the AWS website bellow.
https://aws.amazon.com/marketplace/pp/B01NCXP3K4?qid=1502909952260&sr=0-7&ref_=srh_res_product_title#prod-includes
Regards,