I think I have read that a 2'x4' footprint should be the bare minimum for a single box turtle, so thats 8sq ft, but that was regarding an indoor set-up. So for you 9boxies, 72sq ft total. You could house 4-5 adults in a 4'x8' outdoor inclosure just fine, provided you have a good variety of micro habitats, logs, pool, plants, sandy area, sun, shade, leaf pile, deep rich cool soil, hides, ect., but that should probably be considered the very bare minimum, but like Steph said the bigger the better, use as much space as you have or can afford.
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Jeff Benfer
1.0 cinnamon pastel Python regius
1.1 pastel Python regius
1.1 mojave Python regius
0.3 normal Python regius
1.3 Terrapene carolina thriunguis
2.3 Terrapene carolina carolina
4.1 Kinosternon baurii
1.1 Malaclemys terrapin terrapin
2.2 double het albino and anerythristicThamnophis sirtalis parietalis
1.0 anerythristic Thamnophis sirtalis parietalis
2.3 Iowa snow Thamnophis radix
0.2 het Christmas albino Thamnophis radix
1.1 double het cherry erythristic, albino Thamnophis sirtalis sirtalis
1.1 melanistic Thamnophis sirtalis sirtalis
2.0 66% het snow Thamnophis sirtalis sirtalis
1.1 triple heterozygous for amelanistic,carmel, and stripe Pantherophis guttatus
0.1 anerythristic motley Pantherophis guttatus
0.1 butter p.h. stripe Pantherophis guttatus
0.1 carmel stripe p.h. amel Pantherophis guttatus
0.1 amelanistic p.h. carmel,stripe Pantherophis guttatus