Learn Spanish (Latin America): Yo acababa de mirar la margarita; ¡cuántos pétalos perdió! - I had just looked at the daisy; how many petals it lost!

In exclamatory sentences, when you want to say "how many [nouns]!", the word cuánto has to match the gender and number of the noun it belongs to. Since pétalos (petals) is masculine and plural, it becomes cuántos. If there were no noun and you were just exclaiming about the verb ("how much it lost!"), you would use the default ¡cuánto perdió!.

I learned that acabo de means "I just [did something]". Why is it acababa de here?

You are exactly right! Acabo de plus an infinitive means you have just done something. When you put acabar into the imperfect tense, like acababa de mirar, it shifts the meaning further back into the past: "I had just looked."

Why is acababa in the imperfect tense but perdió is in the preterite?

The imperfect acababa sets the background context of the story (your state of having just looked at the daisy). Perdió is in the preterite because the daisy losing its petals is viewed as a completed, finished event in the past that you are now noticing and reacting to.

Is the yo at the beginning of the sentence completely necessary?