just so you know, the problem with your original script was just a result of order of operations. You need for the oInd var to be defined before the comparison of the result, otherwise it will always be assigned to true or false (depending on the comparaison outcome).
while ( (oInd = _root.blah.indexOf("<br>", oInd+1)) != -1) count++;
… you also need to start oInd at -1, so the initial +1 will make 0.