Welcome Riddhi! Any seeds you have will be fine and it is not too late. All of your questions are good ones.
No minimum, store bought is fine, the quantity per pack is up to you, and you post your seeds and participants will select what they want so you don't have to send for specific zones.
To see what is available to choose from that has been posted so far go to the swap page, select 3rd Annual Not From the Seed Rack Seed Swap, go to the bottom of that page and click join the swap. Once you do that you will have a swap dashboard that will help you.
Also, check out this youtube tutorial:
https://www.youtube.com/watch?...
And read the Procedural thread in this swap.
The thread "This thread is for procedural information and questions about how to proceed." in
3rd Annual Not From the Seed Rack Seed Swap
After you do that you will likely have more specific questions and there are a number of swap veterans that will answer so ask away.