number.to_s.reverse.scan(/\d{1,3}/).join(",").reverse

number            #=> 12345678
.to_s             #=> "12345678"
.reverse          #=> "87654321"
.scan(/\d{1,3}/)  #=> ["876","543","21"]
.join(",")        #=> "876,543,21"
.reverse          #=> "12,345,678"