hi timbin
Say you wanted 30000 shares and 20000 gets filled, you can leave your remaining 10000 order.
Only one brokerage fee applies
If the order doesn't get filled then the order gets purged approx 30 days later.
You could also amend your remaining 10000 order to say 40000, and assuming it get filled, you have 70000 shares at the one brokerage fee.