I guess you mean how to enter the data for many solutions of different compositions like in an Excel spreadsheet or CSV file. Then speciate of all these aqueous solutions.
If this is what you mean, there is no way to do that using supported PHREEQC commands. In PHREEQC you can duplicate solutions in the program memory. For instance, if (SOLUTION 1-10) is heading a given solution composition, the program will create 10 similar solutions. This was created to simplify the setup of 1D coupled transport models in soil columns.
Coming back to your question. One way to do what you intend is to write a script using the PHREEQC COM interface (under Windows OS's) using one of the supported languages such as VB in Excel, Matlab. Or using existing interfaces in R or Python.
What you was talking about is what I meant Usually to calculate the saturation index you need to enter all the data at once Some things are incomprehensible to me and this is normal because it is a talk of dealing with PHREEQC program, but with your help in everything I said I will correct it