There's no documented limit.
Users do note a correlation between poorer performance and the number of domains.
You'll see discussion around the community forums e.g.
https://community.esri.com/t5/arcgis-online-questions/what-is-the-size-limit-for-a-domain/td-p/10422...
I've found that once you're in the 1,000s it can be difficult in some apps and these users saw a drop in performance in the 10,000s.
Also relevant for the user experience entering the data.
And for the end use context e.g. there's documented suggestions in Survey123 to keep choice lists relying on domains under 500.
