You may be able to get by with just the two seats and use Asana as you indicate you want to.
I’d probably recommend you use your nonprofit domain name email to start an Asana “Organization” (not a “Workspace” which is not based on your domain name).
You will benefit from paying for either Asana Premium or Asana Business (vs the free Asana Basic).
If Premium or Business, seats come into play. Anyone who logs into Asana using an email with your domain name becomes a “Member” and occupies a seat that you’ll have to pay for. The minimum is two. If you prevent all others from logging in that way, and instead they log in with other email addresses (e. g., gmail.com, or anything other than your domain), they become “Guests” and will not occupy seats. To learn what Members can do that Guests cannot, see https://asana.com/guide/help/organizations/basics#gl-people . If you can afford seats for all your people, then they could all log in with their nonprofit domain email addresses.
See more in the Asana Guide or below, though recognize there are many other types of ways to set up Asana that these sources describe beyond what I explained above, so you’ll want to focus on just the parts that apply to your case.
Thanks,
Larry