Not an exact number, as it depends on your mmr as you say. Worth noting that op.gg has an estimate of your mmr, but it is not your mmr (same for any other sites estimating it).
Getting demoted tiers is hard though, so the idea tends to be a full tier worth of mmr loss before demotion (i.e. b5 MMR in your case). I wouldn't worry too much until you start seeing bronze 4 players show up in your matches.
If I had to make a blind estimate for how many losses that would take I'd say maybe 15ish straight losses, but it depends on what your true mmr is at and the particular matches you end up in and so on, so really just do you best and don't stress too much about tier demotion, since you really need to not deserve your current tier in order to be demoted.