When you buy an ETF, you don't buy a newly issued cupon. You buy it from another market participant on an exchange - hence Exchange traded fund. What you're describing is closer to classic mutual fund. Each ETF will have "Authorized Participants" who make sure that the ETF mirrors the underlying assets.
So the market depth for any ETF should be constant, regardless of the value of the underlying assets (assuming non-extreme values and outside of crashes or rapid movements).
And yes, to be more exact, the market depth for an ETF should be a function of the amount of trading occuring in that ETF, not so much a constant. In any reasonable timeframe it should be constant.