No, you do not need an Opinyin agent licence for every Freshdesk agent you have. The aggregate allowance of the Opinyin agents you purchase is shared across all your Freshdesk agents.
You can choose the number of Opinyin agents based on how many survey invites you think will be sent out each month and what you feel the likely response rate is. For example, if you set Opinyin up to send a survey invite automatically when tickets are closed/resolved, you could take the number of tickets you close per month that you want to get feedback on, assume a response rate of 30% and divide that by 100, i.e. 1000 tickets per month = 300 responses = 3 agents.
We sell it per agent to align it with the Freshdesk licence model but we don't link it to the number agents in Freshdesk because we know people have occasional Freshdesk user agents and agents that don't handle many tickets, so it didn't feel fair to do that.
The Opinyin subscription model is available on a monthly basis so you can always adjust it to suit your needs, or if you are comfortable you know the number of question responses you are likely to get you can take advantage of a 10% discount for an annual contract.