The only hard limit is the number of routers (L1 plus L2 in-area) on a single Ethernet. That is due to the protocol encoding, which has a list of visible routers on the LAN in the Router Hello message. That is limited by a configuration parameter. The spec says that parameter has an upper bound of 255, but that is incorrect; the actual limit is 33, the largest that fits in a router hello message given the encoding of the field in question.
But if you're using point to point connections I know of no hard limit on the router count.
I guess the concern that Steve Davidson mentions at Spitbrook might have been due to the number of cluster aliases, each requiring an L1 router that's gratuitous from the point of view of wider connectivity, so in a large bridged LAN you could approach the limit more easily if it's only 33.
I could imagine that by the time that Phase IV routing convergence became an issue, DEC's preferred solution was Phase V with IS-IS rather than tweaking Phase IV further.
Trevor