I am not that familiar with Yellow of Parma. However it is not usual for many varieties of onions especially older varieties to have multiple centers. Many of the newer hybrids like Ringmaster are develop specifically for single centers. Multiple center onions will divide into a clump of onions the second year. Most of the older varieties show this multiplier tendency to some degree.

Well that is what I needed to know, thank you farmerdill !! When I plant some of the overwintered ones, i do see the clump of flower stalks and wonder how so many grow. The multiple centers makes sense. And I guess one doesn't really Need single centered onions. It does seem like less centers might contribute to better storage, but it really hasn't bothered ours !!!
Thanks again.
Do you have any experience overwintering small onions planted in pots ? I was thinking of putting them in our crawl space, its about 45 degrees. Otherwise I'm thinking they'll wither away before planting season. Thanks for any ideas...