I came across a very useful section of the BBS website that can give you your intern number sooner than waiting for it in the mail:
BBS MFT Intern Number Verification Section.shtml
1) scroll down to the link "VERIFY LICENSE OR REGISTRATION"
2) Enter your information and press find.
3) Click on your name (if it comes up) and get more info.
Seven days after submitting my intern application my number was available online. It didn't come in the mail for another three weeks.