The first step is to sign up for a free Xero account. Once you have done that, you have two options as to how you can begin development without incurring any cost:
Check out our Development Accounts guide for more details.
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:
Public apps use HMAC-SHA1 for signing and three-legged OAuth 1.0a. The generated token expires after 30 minutes and then the user must re-authenticate to get a new access token.
Partner apps use RSA-SHA1 for signing and three-legged OAuth 1.0a. The generated token expires after 30 minute like Public apps, but with Partner apps the developer can renew the token when it expires without the user reauthenticating.
There's no real benefit to managing multiple connections though, it just makes your token management more complicated. It won't give you additional rate limits.