user18220: What determines how many transactions end up in a given block? Blocks can have various numbers of transactions, so what decides how many are in a certain block? 

user208: Miners (that is, the full node building the miner's blocks - which may be a pool) decide what they put in blocks.
Reasons for including more transactions:

Earn the fees of transactions included.
Make the Bitcoin on-chain ecosystem more useful.

Reasons against including more transactions:

Larger blocks propagate more slowly through the network, giving them a slightly larger chance for being reorganized (as a competing simultaneous block may win).


18220: <p>What determines how many transactions end up in a given block? Blocks can have various numbers of transactions, so what decides how many are in a certain block? </p>
 208: <p>Miners (that is, the full node building the miner's blocks - which may be a pool) decide what they put in blocks.</p>

<p>Reasons <strong>for</strong> including more transactions:</p>

<ul>
<li>Earn the fees of transactions included.</li>
<li>Make the Bitcoin on-chain ecosystem more useful.</li>
</ul>

<p>Reasons <strong>against</strong> including more transactions:</p>

<ul>
<li>Larger blocks propagate more slowly through the network, giving them a slightly larger chance for being reorganized (as a competing simultaneous block may win).</li>
</ul>
