Office 365 (Outlook 2013/2016/2019/Web) - View Multiple Calendars at the Same Time
You can see multiple calendars at the same time side-by-side or combined into a stacked overlay view to help you see availability across calendars. The amount of calendars is restricted only by the ability to still view them. The more calendars you have, the smaller they appear.
The article explains how to open your calendar in a new window, how to view calendars in a side-by-side view, how to overlay them, and how to remove a calendar from the overlay stack.