What do the number of strings on a gartel represent?
Because I do not wear a gartel, I am not very familiar with the subject.
The idea of a gartel is to make an “extra” (beyond one’s belt and pants) separation between the upper body and the lower body, for purposes of prayer. [In the past, this was more of a necessity, because only a cloak was worn, without there being any separation of undergarments or a belt.] Another reason is to make a special preparation for prayer. See Magan Avraham 91:2.
There is therefore no special need for any “number of strings.”
However, in Chasidic circles some ideas are proposed for the number of strings, generally based on kabalistic themes. Some have 18 strings, recallin “chai” (life), others 26 strings (Havayah), and others 36 strings. There are also other numbers — but once more, I am far from expert on this matter.