The distinction between these two quantity adjectives is the distinction between countable quantity vs. bulk quantity.  The rule: If the noun being modified can be counted (“tins of tomatoes”—how many? Twelve? Six?), use “many”.  If the noun being modified is measured in volume or weight, (how much lemonade—A quart? A cup? A glass?), use “much”.  As for “how ---(much) is that tin of tomatoes?”, because the implied word “money” takes a “much”;  perhaps if you asked “How many dollars (or pennies) is that tin of tomatoes?”, then “many” would apply.  The test of the rule is to ask if an actual number makes sense; if “yes”, “many” applies; if not, “much” applies.  “How --- oatmeal did you eat?” “How --- tennis balls can you juggle?”