How many bytes of data does AES 256 encrypt at one time? 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.