Accumulation of Earned Leave
Accumulation of earned leave depends on the state rules in this regard. There are states that permit accumulation of 60 days of earned leave (EL). Similarly, in respect of employees covered by the Sales Promotion Employees (Conditions of Service) Act, they can accumulate up to 180 days.
Encashment of Earned Leave
Therefore, making encashment mandatory is illegal, but you can permit an employee to encash a number of leaves exceeding the maximum accumulation allowed. Since it is an employee benefit, you can enforce it. However, you should not insist that employees refrain from taking leaves, with the expectation of encashing them at the end of the year, as that would be illegal. Moreover, any refused leave will be added to the accumulation, and the accumulated leave will not count towards the maximum accumulation permitted.