@Anton Koval Thanks for your patience on this.
As mentioned over the offline discussion/private message earlier, I have checked with ACS engineering team on this and sharing the below.
We can connect to 1000 domains in one ACS/ECS instance and team has called in this public documentation as well.
Note: In current documentation it is wrongly mentioned as 100 instead of 1000 I am actively working with content author team to fix it accordingly**.**
Hope this helps, let me know if you have any further questions on this.
Please accept as "Yes" if the answer is helpful so that it can help others in the community.