pgbench has a strange restriction to only allow 10 arguments, which is too low for some real world uses.
Since MAX_ARGS is only used for an array of pointers and an array of integers increasing this should not be a problem, so increase value to 255.
While there, correct an off by one error in the error message when you hit the limit. The highest argument id is MAX_ARGS - 1, but the max number of arguments is MAX_ARGS.
--
Simon Riggs http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services