How much penicillinase is needed to deactivate 125mg of amoxicillin?
As March Ho has pointed out, we could do with a bit more information on this one.

I am going to attempt the calculation for penicillin G, and the necessary information seems to be available.


From here, A Levy Unit (LU) is the amount of penicillinase that inactivates 593 IU (International Units) of Sodium Penicillin G  per minute at 25°C and pH 7.0
From here and here, the specific activity of penicillin G is about 1667 Units/mg
I am assuming your concentrate is 20,000 Levy Units/ml (not 20,000 Levy Units/ml/min, which does not make sense to me)
Therefore 1 Levy Unit will inactivate (593/1667)mg, or 0.356 mg, of penicillin G per minute, pH 7.0 and 25oC
2.81 Levy Units will inactivate 1 mg penicillin G under identical conditions
351 Units will theoretically inactivate 125 mg penicillin G in 1 min under the conditions stated.
Your concentrate is 20000 Units/ml, or 20 Units per microliter.  So you will need (351/20) microlitres, or about 17.5 ul.
This assumes that the enzyme continues at a constant rate given by the specific activity right until the end.  This might not necessarily be the case, as one would expect the rate to slow down as the substrate concentration falls below Km.  Personally, I would use at least a 10-fold excess. 176ul - Seems a lot? 


Please feel free to provide further information, or to edit the answer in any way you feel is necessary