Thanks for posting your questions here.
1) There is no limit to the number of products you can track using our Inventory feature, so you should be able to add all of your products to the Manage Inventory Items section within your account.
2) If you select a large number of items and post or schedule them at the same time, they should all be submitted to eBay at the same time. It is possible that each request will take some time to complete (from seconds to a couple minutes) depending on the number of items you selected and your connection speed though.
However, our system will only allow you to post or schedule up to 250 items per batch so, assuming you will need to post more listings than that, you will need to post them in batches. If you need to go that route and would like all your items to post at the same time, I recommend using the “Schedule” option so you can select the same post time for each of your batches.
If you need any more specific assistance with this, please feel free to contact our Customer Support team directly from under the “Help” tab on our site and we will be happy to continue working with you.