ANSWER:
In mass spectrum, the number of peaks tells about the number of isotopes, while the heights of the peaks represent their natural abundance. Two equally strong peaks in the mass spectrum of bromine indicate that bromine has two isotopes which have almost equal abundance. While, only one peak for iodine at 127 amu indicates that iodine has only one isotope with atomic mass 127 amu.
(Ref. An Insight Into Objective Chemsitry-11, Ch-01, Q. No. 29)