Nothing is guaranteed, you can read the monero white paper Zero to Monero for the details. Depending on your threat model you have to assume it would get probabilistically traced in the future at some point.

Basic operational security ideas:

• Over randomized time intervals
• Using different wallets
• Using different amounts

Enough iterations until your comfortable with the risk level. I.e. is it going to be too much work for whoever would care about this to trace it?

Bonus points

• Deposit in an exchange and withdraw later
• Use atomic swaps multiple times, etc

XMR currently takes 16 outputs per input, where 15 of them are decoys and 1 of them is the true one

Assuming you’re not using any traceable pattern like churning every 30mins (you should split your churns, avoid churning too frequently), then the formula is 1/(16^(churn amount)).

So after 1 churn, there is a 1 in 16 chance (6.25%) that this transaction is yours. After 2 churns, it’s a 1 in 16x16 = 1/256 = 0.39% chance that the final output of the route is yours. After 3 churns, 1 in 16x16x16 = 1/4096 = 0.0244%

The probability decreases exponentially.

Please note though, that even after 3 churns, it’s still possible to figure out a link with you. 4000 outputs way less than the total amount of outputs in the blockchain, and you can likely be traced depending on your threat model. As an example, let’s say you own a darknet market. If you deposit to an exchange, you’ll be one of the few with a link to the output that came out of the market to deposit. The closer you are to the output, the riskier it gets. That example alone might not put you in trouble, but you’ll be put on a list. Now imagine this happens again. A second list is formed, and you appear in both lists. It is already really unlikely that the same person appears twice. Third deposit, you appear again. Now you’re probably the only one with this pattern.

Solution? Either churn a lot so that a lot of outputs are in contact (through being chosen as decoys at some point), so you’re not the only one to regularly have outputs linked with the poisoned output. Depositing outputs with no link at the time of deposit would also be beneficial, but that’s not always possible. That’s considering you got the worse threat model, which is a gov sending you outputs and getting them back (through CEXs). Another good solution: withdraw to DEXs!

It depends on how soon you think XMR will be cracked with Quantum Computing. The more churn the better. A pair of hops and you're good I guess.

It really depends on your threat level for the vast majority of people. It's perfectly fine just to do one transaction. You receive Monero and then you send Monero to the person you wish to send it to with no problems.

I guess it's good to be paranoid. Have 2 wallets. One you use to only receive and send to your second wallet only. Age them before Sending to your second wallet. Always spend from second wallet after aging second time.