user-1: Block 689074 had 4,463 transactions.

Is this the highest number of transactions in a block until now?
What are the things that affect the number of transactions possible in a block?

Few observations based on comparison of two blocks: 666666(left) and 689074(right)
Size of transactions:

Type of transactions:

Outputs:


user48884: 
Is this the highest number of transactions in a block until now?

The highest number of transactions in a block is 12239 in block 00000000000000001080e6de32add416cd6cda29f35ec9bce694fea4b964c7be at height 367853.

What are the things that affect the number of transactions possible in a block?

Besides the block size/weight limit, the primary thing that affects how many transactions can fit in a block is the size of the transactions themselves.
When a transaction is added, it takes up space that other transactions could make use of. So the smaller a transaction is, the more transactions can be included in a block. If we look at the winning block, we see that the transactions in it tend to be very small, measuring 81 (v)bytes on average, compared to the 2-300 vbytes in the blocks that you mentioned.

-1: <p>Block <a href="https://mempool.space/block/000000000000000000033c9d18636bdfbaa96d435f4684d8b6efefdc3f826763" rel="nofollow noreferrer">689074</a> had 4,463 transactions.</p>
<ol>
<li>Is this the highest number of transactions in a block until now?</li>
<li>What are the things that affect the number of transactions possible in a block?</li>
</ol>
<p>Few observations based on comparison of two blocks: <a href="https://btc.bitaps.com/666666" rel="nofollow noreferrer">666666</a>(left) and <a href="https://btc.bitaps.com/000000000000000000033c9d18636bdfbaa96d435f4684d8b6efefdc3f826763" rel="nofollow noreferrer">689074</a>(right)</p>
<p>Size of transactions:</p>
<p><a href="https://i.stack.imgur.com/sFW4G.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/sFW4G.png" alt="size-of-tx" /></a></p>
<p>Type of transactions:</p>
<p><a href="https://i.stack.imgur.com/QEftd.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/QEftd.png" alt="segwit-percent" /></a></p>
<p>Outputs:</p>
<p><a href="https://i.stack.imgur.com/Z8dO0.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/Z8dO0.png" alt="outputs" /></a></p>
 48884: <blockquote>
<p>Is this the highest number of transactions in a block until now?</p>
</blockquote>
<p>The highest number of transactions in a block is 12239 in block 00000000000000001080e6de32add416cd6cda29f35ec9bce694fea4b964c7be at height 367853.</p>
<blockquote>
<p>What are the things that affect the number of transactions possible in a block?</p>
</blockquote>
<p>Besides the block size/weight limit, the primary thing that affects how many transactions can fit in a block is the size of the transactions themselves.</p>
<p>When a transaction is added, it takes up space that other transactions could make use of. So the smaller a transaction is, the more transactions can be included in a block. If we look at the winning block, we see that the transactions in it tend to be very small, measuring 81 (v)bytes on average, compared to the 2-300 vbytes in the blocks that you mentioned.</p>
