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The fs.readFileSync() method in node.js is a synchronous method, which means that it will block the event loop until the operation is completed. When multiple instances of this method are invoked concurrently, they will cause blocking and prevent other tasks from executing.

For example, if there are two or more instances of fs.readFileSync() running simultaneously, the event loop will be blocked until these operations are complete, thus restricting the number of simultaneous tasks that can be executed in node.js. This is because the event loop is the backbone of node.js, and it's responsible for managing all the tasks, including I/O operations, network requests, and callbacks. If the event loop is blocked, it can't serve new requests until it's unblocked, which can cause performance issues in the application.