I was asked the other day: "To get 100 seedlings or so from a cross how many flowers do you pollinate on average. And if something turns up that is great do you repeat that cross once more or do you stop there."
That really depends on what the track record of the seed parent is like. If I was to use 'Sequoia Ruby' for example, I would need to pollinate maybe 20 or 25 blooms to guarantee 100 seedlings since nearly every seed it makes germinates. So it all depends on the seed parent. Sometimes you have to just guess and hope.
I have fallen into a pattern of planning crosses in two year cycles, which is to say that if I want to try a cross and I am very uncertain of its viability, I will just pollinate ten blooms or so and see what happens, If the seeds germinate poorly, or not at all, I don't bother trying to repeat it. Sometimes you look at a large population of seedlings from a cross and you select ten from it and they are all very similar, in which case I wouldn't likely repeat the cross, assuming I got the best it had to offer. If I only got 5 seedlings and they all looked very different and very interesting, I would definitely repeat the cross in larger numbers.