So the maximum number of CCC's (current carrying conductors) would be 20 if using 90° C conductors and if the next size up rules in 240.4(B) do not apply.
T310.16: #10 THHN = 40 amps
T310.15(B)(3)(a): 10-20 CCC's = 50% ampacity adjustment factor
40amps *50% = 20 amps