Edwin,
I think it depends on the topic : product, service, geolocation, etc, but it's a balance between quality and quantity.
The greater the sprinkling of links to big names with big reputations (BBC, NHS, government, John Lewis, B&Q, Google, Amazon, M&S, TfL, etc) the fewer the actual number of links should matter.
Taking the SEO example, two simple links to Matt Cutts and searchengineland.com should be far more valuable than dozens of directory entries along the lines of edwinswonderfulseo.biz or optimisedfortonbridgebydavid.org.uk .
A Kent directory that focused on Paddock Wood, Herne Bay, Biddenden and Dymchurch but didn't include entries for businesses in Maidstone, Canterbury, Dover, Sevenoaks and Tunbridge Wells would lack credibility while one covering the latter would be sufficiently robust to be forgiven for omitting the former.
With my geodomains, I try to cover off links to the local BBC, the local council, the local tourism body and the local BBC travel news site and some national generics like English Heritage and the National Trust, plus maybe a couple of local newspapers and local tourist attractions. Beyond that, I think it's OK to stretch to the quantity angle of more obscure sites ... but only once the qualitative credible links are in place.
I think it goes back to the basic argument that content matters : I think that links to credible big players are the bare minimum.
David