I don't know if this is what you mean by there are approximately 1 x 10^18 or greater different combinations for the αβTCR.
Janeway CA Jr, Travers P, Walport M, et al. Immunobiology: The Immune System in Health and Disease. 5th edition. New York: Garland Science; 2001. T-cell receptor gene rearrangement. Available from: http://www.ncbi.nlm.nih.gov/books/NBK27145/