I suspect that as long as people do not own $850 worth of shares in a single market they can input a request for whatever number of shares their account balance will allow based on available funds. I recommend imposing limits that calculate the additional shares a person can purchase and prevent them from requesting more than that limit. I've noticed some share request that I suspect exceed the $850 limit, and are used as a barrier to push buyers toward a different price.
Additionally, a limit of shares that can be requested at once would also be fair. 500 seems like a good ceiling, as I've seen requests for 5000-13000 recently.