There are limits to the number of API calls that your application can make against a particular Xero organisation.

If you encounter a limit, do not continue to make requests as this may continue to add to your limitation. The most common issue encountered is the 60 requests/min rate limit. If possible, queue requests and allow a few seconds before attempting to make another request.
​

If you have a large volume of bills to export, we advise you break them up into smaller batches e.g. 20 bills at a time to avoid errors
​

A single POST to the Accounting or Payroll APIs has a size limit of 3.5MB. There is no limit to the number of elements within a request, provided the overall request size does not exceed 3.5MB. A single POST to the Files API has a size limit of 10MB.
​

Note: When posting form-encoded xml to the API, the encoded data will be approx 50% larger than the original xml message.
​

However, to ensure you receive a timely response from the API (large inserts can take quite a while), we recommend you look to batch elements in bundles of up to 50.
​

Read: if you have set sending attachments to Xero, there is a restriction on file sizes over 3.5MB

Xero is designed for volumes of up to 1,000 Sales invoices (Accounts Receivables) and 1,000 Purchases bills (Accounts Payables) per month, dependent also on the frequency of invoicing during the month, variability of amounts and the frequency of sales tax reporting requirements.
​

This means you may experience slower speeds in Xero when sending over 1000 invoices per month to Xero.
​

source: https://developer.xero.com/documentation/auth-and-limits/xero-api-limits.