It is possible to add additional layers beyond the 2-6 layers PCB123 offers by default.
Hold down CTL+ALT+L and a Set Layers dialog box will pop up, from there you can check the additional layers you wish to add to your design and whether or not you want them to be Plane layers.
Please note though, PCB123 cannot give you accurate pricing information when you go beyond the default six layers. To get a quote for a multilayer board beyond six layers you will need to use the Full Feature service:
Or you can use the Custom Quote service form located here: https://quoting.sunstone.com/QuoteCustom.aspx