In todays press release announcing the acquisition of Zenimax, Microsoft has also revealed more details on Xbox Game Pass. Currently the subscription service (all three tiers combined) has over 15 million subscribers. This means it has gained 5 million subscribers since April this year.
From the Microsoft News website:
As an industry, gaming is expected to be worth more than $200 billion in annual revenue in 2021. Games are the primary growth engine, and they are fueling new cloud-gaming services like Xbox Game Pass, which has reached a new milestone of over 15 million subscribers.