you will need to make multiple calls to get all 700 invoices, via the page parameter in the GetInvoicesAsync. We are working to make the documentation better for all SDK's - but if you look at that function code you can see the 20th optional parameter is int? page = null,
GetInvoicesAsync (
string accessToken,
string xeroTenantId,
DateTime? ifModifiedSince = null,
string where = null,
string order = null,
List<Guid> iDs = null,
List<string> invoiceNumbers = null,
List<Guid> contactIDs = null,
List<string> statuses = null,
int? page = null,
bool? includeArchived = null,
bool? createdByMyApp = null,
int? unitdp = null
);
Since these files are auto-generated then can get quite large, and while it is not ideal to parse through you can find the function def here:
https://raw.githubusercontent.com/XeroAPI/Xero-NetStandard/master/Xero.NetStandard.OAuth2/Api/AccountingApi.cs