It seems strange that there were 8 or 10 FTLC's complete with manifolds, but not more. The interception at Alramal documents that a Manifold is small. (by starship dimensions.) If you were going to ship 400 or 500, why not 1,000 or 1,500?
The Rish appear to play the long game and would not put almost all their manifolds in one basket.
Two shipments of 1,500 each are safer than one of 500 and one of 2,500. No?
I wonder if the yard at Diyu built them, but had quality control issues and the Rish decided to do the hard task.