For heat pumps more outdoor units is definitely more efficient, since the outdoor unit needs to be big enough to accommodate all connected indoor units at once, and will be on even if only one is in use. Two per unit is my go to trade off on that one, with an attempt to group together units that are more likely to be used at the same time (so no bedroom / living room split, try to stick with bedroom/bedroom and living room/dining room etc).

BTW they have low profile indoor units that can be embedded flush with the ceiling or wall, so the bulkiness indoors is not even required anymore.