Apple recommends that you use a dedicated Apple Pass Certificate for each of your use-cases. I.e. if you were running 2 different loyalty programs (with a different purpose) and 1 coupon campaign, then Apple recommends that you use 3 different Apple Pass Certificates, one for each use case. The reason for doing this is so:
Passes don't stack up behind one another (making it hard for the user to retrieve and see in their wallet app)
The icon that appears on the iPhone lock-screen is the correct one (if you share certificates, icon images may be incorrect as it may use the icon image for a previous campaign)
It is perfectly possible to use the same Apple Certificate for all your projects, but you will run into the above issues if customers have multiple cards installed. If this is not a problem for you then you can go ahead with one certificate (it's certainly a bit less overhead to manage as Apple Pass Certificates need to be renewed yearly).
One very important point to note, for agency type businesses where you working with multiple vendors or clients:
To issue Apple Pass Certificate you need to sign up for an Apple Developer account. According to the Apple Developer T&Cs (which you will have to agree to when creating your Apple Developer account), the issuing entity (i.e. the brand -or company that issues and can redeem the passes) needs to be the owner of the certificate. This means that if you want to fully comply with the T&C's, each of your vendors or clients would need to have their own Apple Developer account and Apple Pass Certificates.
This can be very challenging and potentially put up quite the barrier to entry for your clients and vendors (depending on the type of businesses you work with). While we strongly recommend that you always follow the official Apple T&Cs and guidelines, there might be some workarounds for you where you issue the passes on behalf off of your clients under your own certificate. You can then ensure to have a legal agreement in place between your company and your client and add a clear legal disclaimer or terms on the back of the pass which clearly state that your company is authorised to issue passes on behalf of your client, and that the passes can be used and redeemed at the clients locations.
Important: Apple might take action and revoke your certificate(s) if they deem that you are in breach of the Apple Developer agreement. If the certificate is revoked passes can no longer be issued and updated for that certificate.