The pH of the water has little influence on the pH of the mash. You need to understand the buffering capacity of the water, which can be low or high at a given pH.
Certain grains will have higher acidity than others and how much are used, along with the hardness of the water (mainly Ca, and to a lesser extent Mg) combining with phytin from the malt, will lower the mash pH.
So it is not simple, and we can't answer the question as stated. Programs such as Brunwater will solve the problem for you, if you know the water report and the grist for the recipe.