bcos although phosphoric acid also swells cellulosic fibre and same as the case with the soda ash they both make di sodium hydrogen phosphate. please suggest can it prove to be a better alkali than soda ash bcos this formulation will give lower cost as compare to soda ash as the formulation becomes concentrated on reactiON with acid

It does not really matter what chemical you use to adjust the pH. I cannot tell you how much to use. What you must do is use pH paper or a pH meter to determine, by trial and error, how much of these chemicals to add in order to obtain the correct pH for the dye which you are using.

Many fiber reactive dyes work best at a pH between 10.5 and 11.5, but you must check with the manufacturer of your dye to find out which pH you should be striving for. See also what is the effect of pH in dyeing? and what is soda ash used for?.

If the question merely concerns the effect on the fiber, and not on the dye reaction, I cannot help you. I know less about fiber chemistry than dye chemistry. The book "Cellulosics Dyeing", edited by John Shore, has some information on this subject.