For samples with the same number of atoms (and undergoing the same type of decay) yes.

The activity means the number of times an atom decays per unit of time and the half-life is the time in which half of the atoms in a sample decay.

If you have the same number of atoms and you don't consider any possible subsequent decays, yes.

If you store a sample of initially pure Sr-90 then over a week or so its decay product Y-90 will accumulate until you get one Y-90 decay per Sr-90 decay.

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Yes. Radioactive decay can also be expressed by a decay constant, λ, which is effectively an instantaneous decay rate (relative to the amount present).

λ=ln(2)/t1/2.

It gets a bit more complicated with Y-90 since it is itself a decay product of Sr-90, so it will be produced at the same time it decays.

Is this Y-90 accumulation where you get one Y-90 decay per Sr-90 decay similar to saturation in pharmacokinetics where at 5 half-lives the levels are 'stable'?

Assuming Sr-90 and Y-90 are pure beta emitters, does this mean that if I start with pure Sr-90 at x Bqs, given some time I'll actually end up with more radioactivity in the system, as in the combined Bqs of Sr-90 and Y-90 will be greater than the initial radioactivity of pure Sr-90?

Could be similar to pharmacokinetics but I'm not familiar with the use of saturation there. You get this effect whenever you have an (almost) constant production and a decay that's proportional to the concentration. If we look at something like a month then Y-90 production is almost constant because Sr-90 decays slowly over decades. Y-90 decays are proportional to the amount of Y-90, so you start with almost nothing and approach an equilibrium within a few times the half life (~1 week).

If you start with a pure Sr-90 sample then its overall activity will double in that time frame, yes.