Thanks for reaching out about this.
What a great idea to put a sales counter on your product pages! You're correct there in thinking that there are some third party apps available on our Shopify App Store here, that would allow you to add this functionality onto your storefront. I did a bit of search of such apps and found the following that should be able to help you, feel free to take a look at them:
Please note however that these apps are made by third-party developers outside of Shopify. So if you do have any questions about them I would recommend getting in touch with the app developers themselves, as they'd be best equipped to answer specific questions to do with their apps.
Adding the sales counter functionality would hopefully increase your store's conversion rate, as it shows social proof and puts confidence into your customers that others are buying your products! It sounds like this is what you're looking to do for your store at the moment, so I thought I'd include our Conversion Rate Guide here that details more strategies your store can take to optimize your conversion rate.
Let me know how you get on with the information I've provided above! I'm happy to answer any other questions you have.