Dear friend so far Ampicilin is not so efficeint against Staphylococcus aureus. If use it need to be very high concentration which may be problematic for body metabolism. Amp, P, Oxac also cannot be good choise

You can start with a 10^6 CFU/mL culture and treatment time (important) of 2-3 hours. Higher inoculum sizes or less than 2 hours of ampicillin treatment, you might not see any significant cidality. Once you get the killing, you can then optimize it to suit your experimental conditions (medium, strain background etc.)

Ampicillin has a MIC of 0,25 ug/mL in S. aureus. 

@David, im not asking about MIC, i want to know what would be the maximum microbial load it would be able to kill

I agree with Krishan, you can use 10^6 CFU/mL (McFarland of 0,5)

I believe that anything above  108 CFU/ml will be too heavy an inoculum.

@F.E. Emele will it be too heavy even for an antimicrobial substance?

Yes