An unlabeled transitive digraph with 
nodes is called a "topology." The numbers of distinct
topologies on 
,
2, ... nodes are 1, 3, 9, 33, 139, 718, 4535, ... (OEIS A001930).
No larger values are known.
Digraph Topology
See also
Directed Graph, Transitive DigraphExplore with Wolfram|Alpha
References
Sloane, N. J. A. Sequence A001930/M2817 in "The On-Line Encyclopedia of Integer Sequences."Sloane, N. J. A. and Plouffe, S. The Encyclopedia of Integer Sequences. San Diego, CA: Academic Press, 1995.Referenced on Wolfram|Alpha
Digraph TopologyCite this as:
Weisstein, Eric W. "Digraph Topology." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/DigraphTopology.html