The new businesses' venture.
Does this example require an apostrophe with businesses.
What would be multiple businesses if this is just one businesses' venture. (Would you have to say: the group of businesses venture, for instance?)
There's no special rule.
If it's singular you add 's to make the possessive: the new business's venture.
If it's a regular plural (in -s or -es) you add the apostrophe to make it possessive: the new businesses' venture.
Why we do this is a historical accident. Business's, businesses and businesses' all sound identical, and we manage perfectly well in speech with this potential ambiguity; but custom decrees that when we write them down we have to work out whether we mean singular possessive, plural, or plural possessive.