How Many Number of Subdomains are Allowed in SERP Shaker/ATM Sites?

By April

YouTube video
During Episode 58 of Semantic Mastery's Hump Day Hangout, one question asked was about the number of allowed subdomains for SERP Shaker/ATM sites in the IFTTT network. The participant was also asking about its potential impact on indexing.

The exact question was:

For Serp Shaker/ATM type of sites, how many subs/domain is safe? Is building 10 subs per domain too many? Does having more subs effect the indexing??