Your buyback entitlement will be mentioned in your tender form based on the entitlement ratio. The entitlement ratio will be calculated based on the share price and retail shareholding pattern as on record date.
You are assured a buyback for your entitlement, but if you tender more shares than this, the acceptance ratio of the additional shares will be decided based on how many people apply for the buyback.
So taking your example, if you hold 100 shares and if your buyback entitlement is 59 shares, then Infy will buyback the 59 shares from you for sure. But if you tender more than 59 shares, then whether these additional shares will be accepted or not will depend on the number of people and the quantity they apply for in the buyback.