Simultaneous executions limit in google apps script

I think it's per account. This is important when publishing web apps. When set to execute as "Me", you're limited to 30 simultaneous executions of all users, since they all run as "Me". But if set to execute as "user accessing the web app", then each user gets 30 simultaneous executions. Refer @Tanaike's answer

This also makes sense in case of add-on, where a single script project is published to execute as many users. If simultaneous executions of 30 applies per script, then a add-on could only have 30 users, which is certainly not the case.

Related Error messages from Google documentation:

• Script invoked too many times per second for this Google user account. This indicates that the script began executing too many times in a short period. It most commonly occurs for custom functions that are called repeatedly in a single spreadsheet. To avoid this error, code your custom functions so that they only need to be called once per range of data, as explained in the guide to custom functions.
• There are too many scripts running simultaneously for this Google user account. This indicates that you have too many scripts executing at once, although not necessarily the same script. Like the exception above, this most commonly occurs for custom functions that are called repeatedly in a single spreadsheet.

Notice both error messages end in "for this Google user account"

Comments from Google/Issuetracker links:

• https://issuetracker.google.com/issues/168987850#comment6
• https://issuetracker.google.com/issues/36764854#comment8
• https://issuetracker.google.com/issues/144888046
• https://issuetracker.google.com/issues/161091247#comment2

In almost all cases, Google representative says "for account". You may also create a new issue in the issuetracker for explicit documentation or clarification.