My Boss BR-800 came with Sonar Le CD but I can't find the registration # to activate

Did you purchase the Boss device new?  If not, that might explain things.
This may be obvious, but take a good look at all of the documentation.  Wouldn't be the first time a serial# was put in a not-so-obvious place.  This includes looking at any Readme docs.
 
If still no luck, you should contact the dealer you bought it from.  They should be able to accommodate you in some fashion without resorting to contacting Boss/Roland support (which of course is also an option).

I don't think there is a number until you open an account with Cakewalk. 
I think they crank out those disk by the hundreds and a lot are probably never activated. So putting serial numbers on the actual disk is a waist of time and management. 
I just went and looked at my disk and there's no number on it. I also just looked in the owners manual for my Roland A49 and there's no mention of Sonar. 
It might have been a separate sheet of paper I tossed out. But it probably told me to go to the cakewalk site and register.
So create an account and they will E mail you the number. The number is now listed in my store account. 
 
Here is my original E mail they sent me. 
 
Please do not reply to this email. Messages received through this address are not processed. If you need additional assistance with product registration please contact our customer service team at 1-888-CAKEWALK or http://www.cakewalk.com/Support/contact/csr.aspx 

Thank you for registering this copy of SONAR X1 LE. Your registration information is below.