Those who start tying Rambam tzitzit – both if you are tying techelet tzitzit or all white – may be undecided whether to tie 7 chulyoth or 13. The Gemara states very clearly that seven is the minimum and 13 is the maximum. Some poskim hold that 7 is the minimum and 13 is the ideal. According to Rav Avraham ben HaRambam, 7 is the ideal (see (Hamaspik L’Ovdei Hashem, ch. 33).
I have tried to determine which number is predominant among Yemenite Jews who have a mesora for Rambam tzitzit (see here and here). From what I’ve gathered, it seems both are practiced, and there seems to be a division based on the region in Yemen.
Rambam tzitzit with 7 chulyoth
If you wear Ptil Tekhelet and you tie Rambam tzitzit with seven chulyoth, you will be left with a lot of tekhelet string to cut off. That’s why Ptil Tekhelet makes a set called “Rambam 7,” which is shorter.
If you tie Rambam tzitzit, don’t forget that the tied section of the tzitzit must be at least 4 etzba’ot (8-9.6 cm, depending on which opinion you follow). If you tie the chulyoth too close together, they will come out to less than 4 etzba’ot. Of course with thin tzitzit, it’s harder to make the tied section long enough.
On the other hand, if you tie Rambam tzitzit with 13 chulyoth you have to be sure you space them close enough to one another so that you wind up with the hanging section (referred to in halacha as the anaf) is at least two-thirds the length of the section with the chulyoth.