By default, that API endpoint is paged and the amount per_page is set to 10. You can see all the defaults for that endpoint here https://woocommerce.github.io/woocommerce-rest-api-docs/#list-all-orders
If this endpoint works like WP_Query then you’ll need to set the per_page parameter to -1. Otherwise you’ll need to make multiple requests, incrementing the page number each time until you’ve received all the orders