The transaction confirmation speed is proportional to the number of nodes.？
If you mean MINING nodes, then not really.
A block has a difficulty and a gas size. If it takes too much time to mine, the difficulty will decrease and make it easier for miners to mine.
If you have only one miner to mine the blocks (with veyr low computing power), the difficulty will decrease in order to make block mining time to arround 15 seconds.
If you have many miners with different computing power, the difficulty will also adjust.
So the number of mining nodes has an impact, but a small one and only during a reduced time period.