pH of baking soda in water?
1 g of sodium bicarbonate in 400 mL of water will give an approximately 30 mM solution with a pH of ~8.4. The interesting thing about the pH of solutions of amphiprotic compounds is that so long as the concentration of the salt is significantly larger than its Ka and Kb, the solution pH only depends on the dissociation constants. Basically any amount of bicarbonate above 10 mM or so will have the same pH in solution, so pH is not a good indicator of how much acid you can neutralize with it.

I'm not really sure what good this information will do you. How much this solution will be able to affect the paper is going to depend on how much paper you use for a given volume of solution and how acidic the paper is. I don't know much about paper—it could be that it doesn't really matter if you go overboard on the bicarbonate. It might be as simple as making a concentrated solution, testing periodically that your bath hasn't become acidic, similar to photographic stop bath, which has a pH indicator that changes colour when the bath is not acidic anymore. If residual bicarbonate is a problem, rinsing with water afterwards is probably advisable.