How much citric acid is safe to use?
According to (US) Code of Federal Regulations on Citric acid:


  (c)  In  accordance  with  § 184.1(b)(1),  the  ingredient  is  used  in  food  with  no  limitations  other  than  current  good  manufacturing practice. (d) Prior sanctions for this ingredient different  from  the  uses  established  in  this  section  do  not  exist  or  have  been  waived. 


Basically, you can eat the stuff by the spoon-full if you can stand the sour.
If you're adding a lot compared to how much you think you should have to add, you might consider Phosphoric acid instead. You get a little more bang per mole pH-wise with that, and it too, within limits, is generally regarded as safe (GRAS).