Do I need my own agreement with each airline before their flights show on my portal?

In most cases no, you do not need a separate agreement with every airline before their flights start showing on your portal. Flights normally reach your portal through a supplier or consolidator who already holds those airline relationships, so you sell under their access rather than building your own from scratch. That is why a new portal can usually show a wide range of carriers from the first day it goes live.

There are exceptions, and it is fair to know them early. Some low cost carriers, and some airlines in certain markets, only allow selling through a direct arrangement, so their fares will simply be missing until such an arrangement exists. Certain contracted or negotiated fares also sit behind an agreement of their own, which is why two agencies on similar portals can see different results for the same route. Whether you can add a direct airline connection later depends on your supplier, on the API you are connected to and on the plan you are on, so it is not something anyone can honestly promise in advance.

What you almost always do need is your own agency identity for issuing and settlement, along with whatever paperwork your supplier asks for before moving you from test access to live access. That part is about who is responsible for the ticket if something goes wrong, not about each individual airline.

Before you commit to anything, ask your intended supplier for the list of airlines already covered, check that the carriers your own customers actually fly are on it, and ask what would be involved in adding one that is not.