Do I need my own IATA number to start selling flights on my portal?

No, you do not always need your own IATA number to start selling flights on your portal, but it does change what you can sell and how. Many travel businesses start out by selling through a consolidator or a supplier who already holds the airline agreements, and the tickets are issued under their accreditation while the customer stays yours. In that setup what you need is an account with that supplier, not an airline contract of your own.

If you do hold your own IATA number and your own airline deals, your portal can be connected to those instead, so the fares and the commission you have negotiated are the ones your customers see. That usually gives you a better margin and more control over ticketing, but it also means you carry the reporting and the payment obligations that come with holding accreditation yourself.

What is actually possible in each case depends on the supplier and on the plan you are on. Some suppliers will only open an account for agencies with certain paperwork in place, some ask for a deposit before they will let you issue tickets, and low cost airlines often work on a different basis to the full service carriers. None of that is decided by the software, it is decided by whoever the ticket is issued under.

The sensible next step is to ask the supplier you are hoping to work with to tell you in writing what they need from you, and to check whether your existing agency registration is enough, before you fix a launch date for your portal.