Usually, as much as it will fit. A block is a set of transactions and transactions are confirmed by being included in a block. There is no separate process for confirming a transaction. This is part of the creation and mining of blocks. A block must contain at least one transaction, the coin generation transaction (also called a coinbase transaction, not to be confused with transactions made by Coinbase). A block can contain only the coinbase transaction and any number of transactions until the block weight limit is reached. This means that it can have anywhere from a transaction to several thousand transactions. Unbeknownst to some, there is no hard-coded limit to the number of transactions that can fit in a block. Instead, each transaction has a size in bytes and is an economic incentive for miners to include it in their block. Since miners have limited space for sale to transactors, larger transactions (in size, not in bitcoins!) will have to pay higher fees to be included. In addition, each transaction can have a virtually unlimited number of entries or exits.