How Many Listing Can I Have?

The sky is the limit. I don't think ebay has any set limit. Heck, they want you to run as many as you want. Some store sellers have upwards of 1,000, 2,000, 5,000, 10,000, 15,000+ concurrent listings on ebay. You can have as many as you want.

Most sellers that I run in to typically will have 10 - 20 listings at any given time. You do what you are comfortable with. 

You can sell under multiple ebay seller IDs. You don't need to have all your listings under one seller ID.

One seller I know lists her run-of-the-mill stuff under one ID, and her risque naughty lingerie items under a completely different ID. None of her buyers ever know about the other user ID, and if they do happen to stumble upon it, they would never know that all the items originate from the same seller. 

Another seller I know not only lists things for himself, but also makes a little extra income by being a proxy lister. There are any number of names for the same thing, but basically you are listing other people's items for them. They rely on your professionalism, expertise and knowledge of the online selling realm to acquire for them the highest possible price such that they don't have to bother with getting on ebay and creating a seller account, writing up listings, taking pictures and shipping out things once they sold.