There are limits to the number of API calls that your application can make against a particular Xero organisation.
If you exceed either rate limit you will receive a HTTP 503 (Service Unavailable) response. For a full list of API limits, pleace check our API Limits page
You can do more than one thing in a single request: For example, you can create more than one Invoice in a single PUT or POST Invoices API call. While there is no upper limit in the number of nodes that can be sent at one time, a ceiling of about 50 nodes per request is practical - this will ensure a request does not exceed the maximum size of 3.5MB. You should also review our notes on summarizing validation errors.
If you are hitting rate limits because you retrieve a large amount of data from Xero there are couple of features you should be taking advantage of: