I wrote specs for years. Others coded from my specs, so I can take a stab at your question.
SHOP has a lot of code that most clients don’t ever see. They provide a lot more than tools with which to build a customer facing user interface. They have to keep very detailed accounting of every transaction. This is used to calculate part of their revenue, it has to be spot-on accurate, protected from tampering, auditable and reportable (should disputes arise). They support multiple spoken languages (presence in 175 countries). They support building stores on multiple platforms (AMZN, EBAY, etc). They support social media integration. They manage all the shopping cart and checkout functions. They provide the shipping interface. They provide analytics for the shopkeepers. They are expanding into AI driven lending. And the list goes on . . .There’s a lot more they do and they for sure have a product road map of features and functions for future enhancement. They support multiple devices (PC, tablet, phone) and multiple browsers.
You need to understand that it’s not just code jockeys. There are spec writers, DBAs, testers, change control and configuration management folks, security specialists, language experts for localization, social media integration, documentation folks, training material developers, project management, and a lot of hardware and network people are involved involved.
In other words of their current 1,900 employees, 500 or more techies would not surprise me at all.
Before I retired I worked on a project that was basically the purchasing application for a Fortune 50 company (this was in the days before you could buy a COTS MRP product. The project encompassed four primary functions: 1) purchasing functions (RFP, PO, PC, etc.). 2) Shipping (mode, route planning (a lot of oversize/overweight stuff), pricing, schedules, etc.). 3) Quality Control (later known as “Quality Assurance”, inspection planning, shop paper generation, inspection routing, inspection criteria, inspection results, grief process, etc). 4) And Accounts Payable (invoice/PO reconciliation, discount calculation/application, check writing, etc.).
This was quite a few years ago, so my memory is not precise, but I’m certain there were well over 200 techies dedicated to the project during development. Once the project moved to production, the support staff was a lot smaller. But SHOP never finishes development.