Unknown. There is no magic number of concussion allowed before not returning to play. You will be at greater risk of another concussion since you have had so many. Each concussion should be evaluated and monitored by a doctor familiar with concussion treatment. If each concussion has completely healed without any residual symptoms you can discuss returning to full contact sports with your doctor.
Evaluation. More and more is being learned about concussions. Obvious symptoms may include fatigue, nausea, blurred vision, pain upon light exposure.And headaches. Subtle symptoms include " muddy thinking", slow reflexive response, poor school work etc.Final resolution may take months. There are validated tests and specialists for proper evaluation and they should be used.These tests will show true healing.