designing Low-Pass Transmission Line Stub Filter
Yes, the order of your filter (i.e. number of sections) is determined by the stopband attenuation.

You may use the formulae from reference books (1), but I strongly recommend to use free software tools like (2) for that. You'll save a lot of time.

In order to implement a distributed lowpass filter, you should previously design a LC Chebyshev prototype. Then replace inductors by high impedance transmission lines and shunt capacitors by low impedance stubs according to (3):





The same procedure applies to lowpass mininum cap. elliptic filters.



Say, Z0L ~ 20Ohm and Z0H = 400Ohm. YMMV

Hope this helps

[UPDATE]

Example (Chebyshev)


Example (elliptic)


(1) Microwave engineering. David M. Pozar. 4th Edition. Section 8.3

(2) http://www.iowahills.com/9RFFiltersPage.html

(3) Microstrip Filters for RF/Microwave Applications. JIA-SHENG HONG. M. J. LANCASTER. JOHN WILEY &amp; SONS, INC. 2001. page 119. Eq. 5.9