But what does it depend upon?
The price of DCR? Number of trx? How can you be sure it will drop

Sorry for the noob questions
Take a look at the "POS Pool Size" here:
https://dcrstats.com/That's how many live tickets there are. I think (someone correct me if I'm wrong) the target number for that is 4096. So, when the pool size is larger than the target and difficulty adjustment happens, the ticket price goes up. If its lower, the price goes down. Obviously it's more complex than that, but that's the jist of it.
Ok,
thx I think I get it

But if more and more people were to use DCR and participate in the pool (which is what we all believe and hope) than the pool would be larger meaning ticket price would go up?
First some terms clarification: "pool" (POS Pool Size) here means the entire Decred network, not like a mining pool.
But yes, generally more people buying tickets = more live tickets at any given time = higher stake difficulty (ticket price). It's how we've come from a 2 DCR ticket price when PoS went live to where we are today. It also adjusts the other way though too. I doubt for example, that we'll ever see 500 DCR tickets being bought, because people simply won't be willing to pay that much for the benefit (dependent on DCR market price of course), thus causing the stake difficulty to always come close to some equilibrium.