Some customers would like to merge more than 1 PCB in the same gerber to save money, but this may make it hard to cut the board outline and more importantly this will take a lot more time to pick up and package the PCB. Although by doing this, you just have the one order, this complicates the fabrication of the panel and separation of the individual PCBs, so we will usually charge more for this. Similarly, using holes or slots as breakoff sections between boards make them have their own board outline, this will be treated the same way as putting more than one design on a panel.
What will be regarded as different designs?
Basically, if the traces/ silkscreen in your PCB is different, as long as it can be separated, no matter by v-cut, millings, or drill holes, it will be regarded as different designs. One more thing, sometimes, even all your PCBs look the same, but if there is no any connection to make them into one panel, and it needs to be sent as single piece, it will be also considered as different designs.