According to the Asus the PadFone S (PF500KL) and ZenFone Selfies ZD551KL are the first will get the Marshmallow updates and after those two ZenFone 2 versions (ZE550KL and ZE551KL), the Zenfone 2 Deluxe ZE551ML, and no less than five different iterations of the ZenFone 2 Laser: ZE500KG, ZE500KL, ZE550KL, ZE600KL, and ZE601KL get the software updates.
There is nothing clear that the Taiwanese company will add more devices in the list or they are enough for the updates. Before trying to update your Asus device please verify your handset model number because in Asus phones the model number is very important. If your device carry the same model number then it will be updated if not then it will be a disappointment for you.
The company only revealed the list of the devices but the date of rolling out the updates is still under the cover. We have a general idea that above listed device will get the Marshmallow updates in the first month of 2016.