The first rule of bitcoin is don't talk about the size of your stack 

 Precisely. 

 Nosy uncle: how much of those bitcoins do you have then? 
- Not enough.  

 Ask them to show you their bank accounts then you'll show them your wallets.  

 You say you don't have enough.

I also tell people who ask, that I view their question the same as asking them how many dollars they have in their bank account.  

 “What’s bitcoin?” 

 Most grownups don’t ask questions like how much do you make?
I feel like this is the same caliber of question. 

 “trading to grow my stack” 

 tell them you have 21 million bitcoins 

 Trust no one. 

Tell them you had to sell everything to pay debt

Tell them Bitcoin is freedom, but, unfortunately, you did not take care of self governance and lost your keys. 

If they have your address, for whatever reason (or you were a fool on googling your address) study CoinJoin or use lightning network to make your coins “vanish” into a new wallet address and tell them you got hacked 

There’s one person that needs to know if you have bitcoin: you. 

There’s only one being that needs to know exactly how much you have: God.