How many IPv4 addresses does the block 10.0.0.0/24 contain?

The block holds 256 addresses. An IPv4 address has 32 bits. /24 fixes the first 24 bits, so 8 bits are left. 2^8 = 256 addresses, from 10.0.0.0 to 10.0.0.255.

"254 in total" is wrong. In a traditional subnet, two addresses are not given to hosts: the network address and the broadcast address. That is where 254 comes from. But the block itself still holds 256. "65,536" is the size of a /16 block, not a /24.