The logic is that the themes have nothing to do with storing the number of listings as the storage happens in the database. Themes have to do everything how it retrieves the data. If it handles a query that runs an infinite loop to retrieve all data at once, instead of smaller datasets (Example: Batch of 20 listings), then it runs into problems such as resulting in crashing the site.
But all our loops are checked and tested to make sure none of them have an infinite loop. We have users with over 300,000 listings and we have even tested with over a million listings as well without a problem.
For optimal performance, a low cost shared hosting is never recommended for running a real business directory site with thousands of listings. What you want to consider is a VPS or Dedicated Hosting.
Purchasing a low cost shared hosting is never recommended for running a real business directory site with thousands of listings. What you want to consider is a VPS or Dedicated Hosting (Read More).