Since the block size of AES 256 is 256, how would we calculate the bytes of data AES 256 encrypts at one time?
AES has always 128-bit block size with 128,192 and 256-bit keyspaces. Therefore, you can encrypt 16-byte at a time if you are using ECB and CBC modes. By using CTR mode you can encrypt 1-bit to 128-bit.
Some people confuse AES with Rijndael where AES is a variant of the Rijndael. Some old libraries use the Rijndael (see RijndaelManaged). Rijndael can have 128, 160, 192, 224, and 256-bit block size. Note that these are not standardized, see FIPS 197.