now I just pitched yeast for my last batch, and I noticed that I have
one extra package of yeast. Now I could let it sit in the fridge for
the next year until the plums ripen again, but I really dont want to
worry about it for that long.

I am betting some object in the fridge will puncture that yeast
package in the next 11 months, and yeast is cheap, why should i make
so much effort to save it?

just about all instructions say one package for every five gallons,
but can you add too much yeast?