Hi Unnurgudna and welcome to the forum,
Just let me start by clarifying that drop-shipping and wholesale (hole sale) are two completely different supply methods. Drop-shipping provides you with the opportunity to sell products on behalf of a supplier on a per item basis, essentially you act as a salesperson for that supplier. Wholesale requires you to purchase in bulk, meaning that those products then belong to you.
It's very common for new sellers to confuse the two, but they are very different on many levels, that's just the basic difference I have given above. So the very first thing you really need to do is decide which supply method is going to be best for you. Of course buying in bulk requires at least some level of cash flow, so if you don't have that behind you, then drop-shipping may be the best option for you.
Once you know what supply model you are going to use and the sales channel/s you wish to target (in this case it seems to be eBay) then you need to start researching the markets. You need to start identifying product/s that you are interested in selling, then researching those particular products within that sales channels to see what your competition is like.
When you know what your competition looks like, then you can start researching suppliers for those particular products and see what sort of a competitive advantage you can find to see if that product/s are viable in that market.
In regards to how many you list, well that's really a personal thing. One of the advantages of drop-shipping is that you aren't out laying cash up front, so it affords you the ability to be far more flexible with what you can do. Just bare in mind that there may be listing fees to consider, so even though there may not be any products prices up front, there can still be marketing costs you will need to cover.