Short answer
HubSpot's KB: limits run on a rolling 24-hour period. Free accounts get 500 emails per day. Providers have their own limits HubSpot can't override. Sales Hub Professional or Enterprise can set a custom daily limit.
1. In HubSpot
The relevant screen in my demo portal.
2. Worth knowing
Hitting a provider limit can happen below the count, since bandwidth limits also apply.
3. Related
One-to-one emails and sequence settings.