Can I estimate time to confirm based on the number of existing transactions awaiting confirmation?
This is an impossible question to answer, because there will likely be new transactions added to the mempool continuously. 


If more transactions get confirmed than are added, the mempool will shrink in size, and lower fee txs will confirm.
If less transactions get confirmed than are added, the mempool will grow in size, and it is likely that higher fee txs will confirm. 


As a user, the total number of transactions shouldn't be of concern to you, instead you should look at what the fee rates of currently confirming transactions are, and plan accordingly. If your tx needs to be confirmed ASAP, you should set a fee slightly higher than what you are currently seeing. If you can afford to wait, you can set the fee lower, and hope that the mempool will shrink in size (but this is fundamentally unpredictable, it depends on the actions of all other useres).

Ultimately, you'll really only care about the number of txs paying a higher rate than you are willing to. Those txs are the ones that will affect how long your tx takes to confirm.