You can contribute as much as you want per year to an RESP, but no more than $50,000 in total per beneficiary. Neither government grants nor any investment growth in your RESP count toward the $50,000 limit, so there could be more than $50,000 in your plan by the time your child needs it.
You can contribute to an RESP at any time during the year, as there is no deadline. Government grants are applied by the calendar year.