There is no limit on how many different Coda docs you can have open at one time. However, Coda does not currently allow you to open the same doc in multiple tabs but we plan to allow this in the near future.
For those using Chrome or any browser with a Chrome web store (ex. Opera, Vivaldi, Edge Chromium, Brave): You can Install the Coda extension and it will automatically redirect you to the existing tab so you don't have to look for it.
The extension is not currently available on Safari. You can still use a doc in two windows by opening it in two different browsers (like Chrome + Safari).