Short answer
HubSpot's KB: up to 25 property change events per account. They count toward the 30M monthly event limit and occurrences past it aren't recoverable. HubSpot shows each property's fill rate.
1. In HubSpot
The relevant screen in my demo portal.

2. Worth knowing
Pick properties that change meaningfully, not every update.
3. Related
Codeless events and analyzing events.