The limit seems to be 2-7171922938 [1][2] for some reason 1 throws an error [3].

Edit: I should have read the documentation first [4].

> The current database contains 7,171,922,938 agents and is approximately 6.8 TB in size.

[1] http://pplapi.com/2.html

[2] http://pplapi.com/7171922938.html

[3] http://pplapi.com/1.html

[4] http://pplapi.com/docs/