The number in parenthesis is the number of their co-authors (as calculated by Mathscinet in 2013). Complementary source, Google Scholar Citations. You can also consult all the co-authors of regular members of Lacim on this web page.
Just for fun, at least 6 of my co-authors have Erdös-number 2, so that my own Erdös number is 3. My average distance to the 51 (out of 52) Fields Medalists that appear in the math collaboration graph is (a small bit less than) 4. Considering this average distance might make more sense than the Erdös number, if one insists on using (to what purpose?) such exaggeratedly reductive metrics.