Put the summary of the bug you're having here
What I expected to happen was...:
Getting 40 HP
What actually happened was...:
Going up to about 30, and then the game crashed.
Steps to Reproduce:
1. Run /effect give @s health_boost 1000000 9
2. Give yourself Regeneration level 7 (so put a 6 in the command), and wait.
Incomplete without a crash report from (minecraft/crash-reports/crash-<DATE>-client.txt). Please attach it to this ticket so that we can diagnose your issue.