why sometimes block has 16 transactions and sometimes over 2000? I checked it on https://blockexplorer.com/blocks. Thank you
Since the block size is fixed (but has changed since genesis block) there are 2 main reasons:
- Transactions are not all the same size in bytes. One transaction can have multiple inputs and outputs, increasing the size of the tx in bytes.
- The market might not fill a block with transactions (esp. in the beginning blocks) because usage is low.