De acordo com o código-fonte localizado aqui, começando na linha 850, o PostgreSQL não limita explicitamente o número de argumentos.
O seguinte é um comentário de código da linha 870:
/*
* We try to generate a ScalarArrayOpExpr from IN/NOT IN, but this is only
* possible if the inputs are all scalars (no RowExprs) and there is a
* suitable array type available. If not, we fall back to a boolean
* condition tree with multiple copies of the lefthand expression.
* Also, any IN-list items that contain Vars are handled as separate
* boolean conditions, because that gives the planner more scope for
* optimization on such clauses.
*
* First step: transform all the inputs, and detect whether any are
* RowExprs or contain Vars.
*/
