I think I asked this same question a few years ago, and there didn"t seem to be any verifiable answer. The short answer is, not many. I had an IH collector tell me that the general estimate was around 1400 units, in the Hi-utility.
We bought one that had been sitting outside for 10+ years, and the block and head were cracked. Parts are indeed hard to find, but you sometimes you get lucky. Ours currently has a 303 combine engine in it and really runs great.