All the items forever! Unless you have a strong PR campaign directing the buyers to your store and/or a strong buyer base built-up to allow you to release your items in small, curated batches as a "line" for others to purchase you need as may need hundreds of pieces of merchandise in your online shop!
Likewise, a craftsman, artist, or knowledgeable reseller can sell a smaller amount of items by focusing on keywords, valuable vintage items and demographics, but I think even this type of focused seller can benefit from having lots of items and diversity available.
Here's some advice from long-time online sellers on the subject:
"I think it depends on what you have to sell and the demand for it."
-Sew Inspired, Blog
"From what I've read regarding etsy... you should list at least 300 items to make sure you're driving enough traffic. Also, you should not do "batch" listings. You should list something everyday. If you do batch listings most of your items just end up unseen. And your keywords are exactly that - KEY! Don't just use the same words from your title.
That's my advice from almost 4 years on etsy (with 2 shops) and 7 (or 8?) on eBay."
"It really depends. For the type of items I like to sell on eBay I can COUNT on making a profit of $1 a month per listing OR MORE. My worst months it is still the same. 250 listings= I can count on clearing $250. Most months it is more than that. I will work on getting my number up to 500 by the end of the year and will really be focusing on this shop more in the coming months. "
Each item you list is another way for a shopper to find your shop, think of each one as a low-priced ad. Each item listing has the potential to get eyeballs and potential customers. The more, the better.
Look at your online shop. If sales are not where you want them, list more items. Do it. That is my professional opinion.
This is a numbers game, if you want to earn a lot more you need to get more out there. I'm working on getting $5K worth of merchandise into the world right now!
What do you think? Do you have a certain number of items you like to keep in your online shops whether it's eBay or Etsy, vintage or not? Let's discuss in the comments.