Wp Users Amount

Want to ask? How many Users can the Wordpres Database take. I run a web site an i currently have 10,000 members and just recenttly my database crashed, how do i prevent that from happining again and how do i send mail to this 10,000 and increasing number of memebers all at once or in bacthes.

Pls Any suggestion on plugins or softwares to send mail to that amount of users,

Thank you.