Yes, the issue number on a white card is not a Personal No. It merely shows the consecutive issue number from the immigration office in your region. Mine only had a single digit as it was a pretty quiet office...
They probably didn't give you a printed document with your white card when you collected it. But you can go into your local immigration office and get them to print out the correct information, including your Personal No. (I can't remember if they make a small charge or not).
But while you're there, why not get yourself the new-style card as well, which is far more useful?
The process is quite simple and it costs either 18 or 36 leva, depending on the processing time requested, plus, of course, a few leva for the bank's commission.