Yes you can get a buy to let mortgage on a multi-unit property, however not all lenders will consider lending on this type of security as it is not a property type that can be easily sold to a single family. An example of a multi-unit property could be a single house which has been converted or split into two of more units, but the overall property is on a single title.
The lenders that usually accept this property type, will require for the individual units to be self-contained with separate access, own utilities, and they must not have individual leases on the units, if you are looking to lending against the whole building.
Some lenders will also require a minimum amount of landlord experience, before they will consider lending on this property type.