How high will the ck-mb be elevated if there is a myocardial infarction?

Depends: The elevation of ck-mb is in direct proportion to the size of the infarction. The highest i personally have ever seen was about 1000 but that is very unsual. Also, ck-mb will rise as a % of the total circulating ck. Normall ck-mb is less than 1.5% of total ck. After infarction, ck-mb will go to anywhere between 2.5-15% of total ck.

Depends on lab: Each lab publishes its own normal range so check with your lab. More importantly though, is not the absolute level. It's the pattern of rising and falling which is indicative of mi. Some normal people always have a markedly elevated CPK and have normal hearts. Muscle disease or even vigorous muscular work can raise the CPK too.