How much of the starter you use may depend on whether you want to use the starter for leavening purposes or whether you want to use an amount that will be much greater and serve as a preferment and confer attributes of preferments on your dough.
For example, for a naturally leavened dough for a Neapolitan pizza, the amount of starter to use can be, say, 1-5% by weight of water. That number can be converted to a percent of the weight of flour (or even as a percent of the total dough weight) if you have all of the ingredients and weights for the recipe and they are all workable. When I made naturally-leavened Neapolitan doughs, I tended to work at the high end of the 1-5% range. When I wanted to use a preferment approach, I used 15-20% by weight of flour. That range came from a member, bakerboy (not bakerbill), a baker who used that range when he made pizzas (he now runs a bakery). At one point, Jeff V. used around 40%, although it could have been lower depending on how he calculated his percent. pftaylor at one point used around 8% (by weight of flour) for his Raquel dough formulation, but I believe he recently lowered that amount to 5% (peak activity) by weight of flour.
As you can see, there are many possibilities depending on your objective, and it is likely that other members can offer up even more suggestions based on their experiences.