In both cases you have an option in the send mail dialog:
Create: Multiple emails one for each record in found set.
If you get all the records for your intended recipients into your found set (Show All Records, Perform a Find ), you can use this option to send one email out for each record.
Many ISPs and Email providers have limits on the number of addressees and number of emails per hour or day.
They are mostly in the 50 to 100 addresses per email.
See this list