How many people can I send email to at once? I'm sending out graduation announcement and it won't let me. ?

If you try to send too many emails at one time Yahoo will flag you for spamming. Limits on sending mail in Yahoo Mail: https://help.yahoo.com/kb/sln3353.html

No, no, no, no, no! Did your school not give you the opportunity to buy the graduation announcements that you mail?  You don't email graduation announcements- that's cheap and gauche. 

How to Create a Mailing List in Yahoo Mail: https://www.lifewire.com/create-mailing-list-yahoo-1174477