Short answer
HubSpot's KB: users with Bulk enroll sequences permission can enroll up to 50 contacts at once. Bulk sends go out at a maximum of three emails per minute and respect your daily send limits, so the last emails can go out later than the first. For a larger list, enroll in batches or use workflow enrollment where your tier allows.
1. Sequences
The sequences tool in my demo portal, before any sequences exist.
2. Plan for volume
Sequences are for one-to-one follow up from a rep's inbox. For a large list, a marketing email may fit better.
3. Related
Sequence settings and out of office replies.