How many users can Craft handle? I'm looking to use a Craft3 installation as a bridge app to synch a client's contacts from their CRM server to their Email Service Provider, via respective APIs.

I'll be storing their contacts as users in Craft. They have four contact lists with numbers (there may be some overlap) totaling:


12,000 contacts
55,000 contacts
23,000 contacts
22,000 contacts


Is storing these in Craft as users a realistic option? We'll be editing/adding new users in small batches on a daily basis, slicing them off the full list on the source side by last updated date, and then filtering records to push to the destination using the Craft API.
It depends more on how you are going to create and display them. I had to import 20k elements this weekend and I had to be careful to store them step by step. 

Depending on your server/hardware - you can't just insert 10k elements in one request. There is a function App::maxPowerCaptain() to disable the timeout but I still faced overflow errors. 

And of course you can't display so many users in one single page without using a pagination or Ajax

Besides that - Craft is able to handle such an amount of elements if you can do it in your code as well