Is there a limit on what an owner/shareholder and turn in for a monthly expense report?

I'd say the balance in the corporate checking account establishes a practical upper bound :-) OK, kidding a bit, but not sure what exactly you're asking w.r.t. "limit".

Generally, the corporation should reimburse for those expenditures incurred for a valid business reason, and which are for the benefit of the corporation. Should not reimburse for expenditures that are primarily for the personal benefit of the shareholder. The latter would be a form of dividend, which (1) has a different tax treatment for both corp and shareholder than valid biz expenses; and (2) if there are other shareholders holding the same class of stock but not receiving equivalent benefits, they'd justifiably be sort of cranky about it.