Original paper. 

TL;DR: Fewer, but larger and faster dendrites (more efficiently connected) predict higher intelligence. 

The results of this paper and generally understanding what exactly the biological causes of increased intelligence are seem relevant for:

1. Strong human intelligence amplification, especially short-term interventions if germline engineering is too slow.
2. Psychedelics - they promote an increase in the growth of new dendrites. Thus they may be counterproductive for intelligence (at least without adequate pruning).
   1. From Ly et. al:
      (F–I) Cortical neurons treated with psychedelics display an increase in the number of branches (F), the number of primary dendrites (G), and the total length of the dendritic arbor (H) but not the length of the longest dendrite (I).
3. Contemporary nootropics: Psychedelics (see (G) of quote), or compounds like gintonin, might directly affect intelligence by increasing dendrite size, if paired with something that causes efficient neural pruning.