What is the next number in the series? 83 79 75 71 67

This is an arithmetic sequence with a common difference of - 4, meaning that each term is four less than the previous term. So the next term in that sequence (technically the term "series" should be reserve for infinite sequences) will be 67-4=63.

83-79 = 4

79-75 = 4

the numbers are decreasing by 4

so the next number would be 67-4 = 63