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Best practices would be to 301 all those pages to their relevant new page. Now, if you're not worried about the traffic going to some of those old pages you could choose to 404 the page and let it die but you'd be missing out on any link equity going to that page... but if there are no external links pointing to them then you only have internal links to worry about (but those are likely all on old pages which no one will see anymore either via 301s or 404s). As for those products with 5+ URLs on the old site... redirect them to the relevant new page. It doesn't matter that there are 5 URLs redirecting to one if that one page is relevant to all 5 old URLs. I wouldn't worry much about server load either with that amount of redirects and eventually any old listings in the SERPs will update to the newer URL (if relevant for the specific query) instead of constantly sending people to a page they know exists elsewhere.