how many losses does it take to get demoted

it's hard to say. it depends on your mmr. shielding will tipical block one game after that it depend on your mmr witch goes up the more you win and down the more you lose so lets say you lose every game from this point on once you got to 0 lp silver 5 it would probably take 2 game one from shielding and one for the loss to register

At the lowest (5) of any division demotion is very hard

In terms of regular demotion I would say 3 losses is a good number in terms of being demoted.

If you were in Silver 4 it would be stupidly easy to get demoted (speaking as someone who moves up and down the ladder very frequently) but demoting from Silver 5 would take your MMR being on Bronze 5 levels.