Roblox had about 25 million monthly active users in 2015.

Quick Scoop

That figure is usually reported as monthly active users, not the number of people who played every day.

Context

By comparison, Roblox grew to over 120 million monthly active users by 2020, so 2015 was still an early growth stage for the platform.

TL;DR

In 2015, the best available estimate is 25 million monthly players.