From what I know, hyphenated domains are not worth much as long as they are not something ultra premium (like used-cars.com, cup-cakes.com and etc.) But the problem as mentioned above is that the type-in traffic will be lost to unhyphenated version domain.
Secondly, you ask as 'how many hyphens should be there in a domain'? I guess one hyphen is more than enough, considering the domain has pure generic value as mentioned in the examples above.
Just my 2cents.