There’s not really a maximum amount. Instead, if your expenses are too low, you may not be able to claim them. This FAQ has the formula used to calculate how much you can claim: https://turbotax.community.intuit.ca/community/family-health/help/how-does-my-income-affect-how-much-i-can-deduct-for-medical-expenses/00/873498
You can choose any 12-month period of medical expenses, so long as it ends in the tax year for which you are doing your taxes. So if you have a lot of expenses and don’t need them all to reduce your taxes, you can choose a period this year that only covers part of them, and then next year you can claim the rest. This FAQ has more info: https://turbotax.community.intuit.ca/community/family-health/help/can-i-claim-prior-year-medical-expenses/00/873510