TLDR;
We don't set a limit on how many users can use an extension at the same time. However, Airtable API limitations can cause issues when lots of users access your extensions at the same time.
Use Case Example
Can 100 people fill out a form in a minute and nothing breaks?
If 5 of those 100 people save the form at the same exact second, then the Airtable API limit will be reached. If the 100 people would submit the form over the span of a couple of minutes, then there is no risk of reaching the limit, but there is a risk of reaching the limit if 100 people all submit the form within the same exact minute. Cases of limits being reached are very rare, but we advise our users to spread out rollouts to lots of users over a longer period of time, if possible.
Airtable API limit
Airtable's API has a limit of 5 requests per second. That's the only hard limit that exists. In reality, this limit has rarely been an issue with our customers, some of whom have shared their forms and portals with thousands of people without any problems. In our experience, this API limit is almost never hit. It would take hundreds of people all trying to save the form within a very short amount of time for this to be any concern at all.
In the rare case you do hit the Airtable API limit (e.g. when you have many edits at the same moment), some users may get an error -from Airtable- and they'll be able to try again. If the API rate limit is reached, there's a short timeout period set by Airtable before the ability to make API requests is re-enabled.
miniExtensions automatically retry to submit if the API limit is reached. If the request fails after retrying many times, the user gets an error. Users only see a loading indicator on the screen while the system keeps retrying in the background until they see a success message (or in extremely rare cases, an error).
miniExtensions Form
In case you receive extremely high submissions at the same time and you hit the Airtable API limit, the form should still load. Accessing the linked record finder can fail in those rare cases, since this also pulls data from the Airtable API. Some users may get an error when they click submit, but they will be able to wait a few moments and click submit again. The data in their form won't be lost and they won't have to restart.
Our customers use miniExtensions in a production setting every day. We've only seen the API limit becoming an issue when you have hundreds of users updating your data at the same time. So if you don't expect that to be the case, then you have nothing to worry about.