Number of different clones of B lymphocytes
Alternate spicing of mRNA is a real phenomenon, but it's not the source of immune cell receptor diversity.  The DNA of immune cells is rearranged in a slightly random manner, to make a receptor with a random sequence in the variable portion of the receptor.  As Armatus already said, the process is VDJ recombination.  Here is a link to a textbook on NCBI

https://www.ncbi.nlm.nih.gov/books/NBK27113/