Typable terms are terms that satisfy constraints which make them well formed.
The current computation requires one to generate all the plain lambda terms and to sieve out those that are typable. This is feasible for the 63782411 terms of size 10 but not for the 851368766 terms of size 11.
Alonzo Church, A Formulation of the Simple Theory of Types, J. Symb. Log. 5(2): 56-68 (1940).