maternity leave is accountable for bonus calculations?

I was on maternity leave in the Financial year 2017-18, company should include my maternity leave for bonus calculation?

If you were on maternity leave from the company and not from the ESI, then you should have received salary during the leave period from the company.Then why should the company hesitate to pay bonus? Bonus is payable on the salary paid to the employees. So, if you have received salary, you should get bonus also.

I have read something like only below 21000 rps /month salaried persons can get bonus? my Salary structure is BAsic 20000/Gross 61K/Net 58.9K.

Yes you are eligible for Bonus as per ACT
I have read something like only below 21000 rps /month salaried persons can get bonus? my Salary structure is BAsic 20000/Gross 61K/Net 58.9K.

Certainly, only persons having salary upto Rs 21000 will be eligible for bonus. But if the company calculates bonus of employees based on the basic pay only, then any employee whose basic pay is not more than Rs 21000 should be eligible for bonus even though his total/ gross salary is much higher than that. Again, in respect of employees whose salary exceeds Rs 7000 but within Rs 21000, bonus will be calculated on a salary of Rs 7000 or the notified minimum wages of the state whichever is higher. Therefore, if your industry has a notified minimum wages and if that minimum wages is Rs 12000, then your bonus shall be based on that Rs 12000. At the same time, if there is no statutory minimum wages fixed for your kind of employment or if your statutory minimum wages is below Rs 7000, say, Rs 6000, then your bonus qualifying salary will be Rs 7000.

Adding to the observations of our learned friend Madhu on counting of the period of maternity leave availed of by a woman employee during the particular year for which bonus is to be calculated and paid, the number of days which an employee under the Act works in the establishment during the respective AY has no relevance at all for computation of bonus but for the minimum no of 30 days prescribed under section 8 of the Payment of Bonus Act,1965 AND the ONLY unit of measurement for calculation of bonus is the salary or wage earned by the employee during the AY either for minimum bonus u/s 10 or maximum bonus u/s 11 as the case be.