Depends on patient
Depending on what symptoms, diseases, and other features a patient has, a lab value can have different meanings.
The "high" limit on platelets
is often "400". That means about 98% of normal people will be 400 or less, and about 2% of normal people will be over 400. The farther above 400, the more likely the result means a problem. One can visit his doctor and ask what his result means for him.