If it's 32 equal sized loops of 200' running ~0.4gpm each, 13' @ 12gpm sounds like the right ballpark. The distribution lines to/from manifolds are likely to be bigger than half inch, and may add a bit to the total if they're skinny and long, or if there are several tight elbows, etc. . Order of magnitude feels right, for all valves open.
Micro-zoning it into 32 separately controlled zones is a disaster for the efficiency of the boiler though, since it's min-mod output is WAY over the load of boiler, and it will short cycle on zone calls for the smaller zones, even if the radiation is all concrete slab.
"Heavily insulated with good glass", is a meaningless for estimating the design condition heat load, as is the square footage of conditioned floor area. The actual construction type & R-values, U-factors of the windows/doors, (with total exterior surface area of wall/attic/winows, etc) and air-tightness (got a cfm/50 number?), and the 99% outside design temperature are what it takes. But the design heat load could be met with anything from 1gpm to 100gpm, depending on the radiation design and the designed water temp.
At 12gpm with a 10F delta on the radiation you're looking at about 60,000BTU/hr, which could easily exceed the heat load of a well insulated 6000' house in a temperate climate. (Or even a code-min house at IRC 2012 prescribed R values & U-factors.)