How much R410 a freone does it take to contaminate a R22 freone system?

Any amount would be a contamination. Charging with clean R410a does not introduce POA oil, which is where a serious contamination would occur. Mixing R410a without oil will simply render that amount of charge in your system as ineffective. To answer the question how much contaminates, it could be seen on a sliding scale, a very small amount, say a couple ounces, probably will not see a noticeable change. A pound, and you would likely notice some changes in performance related to temps. of evaporator as well as the amount of liquid refrigerant its producing to send to the evaporator. As you increase the ratio of R410a to R22, you would only see incrementally worse performance as the compressor is not producing the pressure required to get any use of R410a. If you had somehow introduced R410a with its oil, their would be a serious reason to pull the entire charge, flush the system, and start clean. The oils of R22 and R410a are %100 not mixable, they will be rendered useless, and system damage will occur shortly.