To which country in Europe should I travel to if I want to see a lot of lighthouses?

Another answer, courtesy of an email response from Russ Rowlett of The Lighthouse Directory:

All the European countries have many lighthouses, but they differ in how accessible they are. The Netherlands has a large number of major lighthouses within a small area, and most of them are accessible. Portugal has many beautiful lighthouses and they are accessible, but not open for climbing. Britain also has lots of lighthouses and there are some famous ones that are open.

Mark's answer mentioned Estonia (with 42 lighthouses); let me focus on that a bit more.

Some friends visited Hiiumaa island this summer, and (through foursquare) I saw them post gorgeous photos of one lighthouse after another. The other big island off the Estonian west coast, Saaremaa, also hosts a plethora of lighthouses.

^{Tahkuna Lighthouse, Hiiumaa, 2012. Photo by Tiina L.}

A thing to note is that these islands are relatively compact (as is all of Estonia), yet have lots of lighthouses. There are good roads and it's easy to get around, at least if you have a car or bike. In summer, bicycle would probably be an ideal way to move around. (I'm not sure about public transport on the islands, but that would make a good Travel SE question...) Also, there's plenty of accommodation and restaurants available, and it's all pretty affordable (for Europe anyway).

Examples of Hiiumaa and Saaremaa lighthouses:

Hiiumaa

• Kõpu Lighthouse. (Pics on Foursquare)
• Tahkuna Lighthouse. (Pics on Foursquare)
• Sõru ülemine tuletorn
• Ristna Lighthouse
• Hiiessaare alumine tuletorn

Saaremaa

• Abruka lighthouse
• Anseküla lighthouse
• Vilsandi lighthouse
• Sõrve lighthouse
• Sääretuka lighthouse
• Loode tuletorn
• Kaavi tuletorn

One good listing of most lighthouses in the county is the 'Eesti tuletornid' category in Estonian language wikipedia.

Finally, this blog post about a Hiiumaa day trip is nice and informative... if you read Finnish, or as you likely don't, just look at the photos. :-)

^{Sõru, Hiiumaa. Photo credit: Tuderna (Panoramio / Wikimedia Commons)}

^{Anseküla, Saaremaa. Photo credit: Tuderna (Panoramio / Wikimedia Commons)}

You can check out the lighthouse listing worldwide at lighthouse directory from the University of North Carolina.

I can't really recommend any specific country for visiting lighthouses since I'm not into that but can only say that you can check out Croatia lighthouses. They are all very different from each other and you can use a lot of them as accommodation during your stay. I've seen around 10 to 15 of them in Croatia all over the coast and the islands and if you don't intend to visit our coast from north to south, then focus on Istria and Kvarner (the northern part of the coast).

Some are near city areas and some are completely isolated on their own islands or reefs.

There are essentially two sorts of lighthouses. Those built on land, usually a group of buildings, with a tower, built on a headland. And rock lighthouses, built on a rock several miles out to sea. In Plymouth (UK) there is the first rock lighthouse, which was originally built on Eddystone rock. This has been rebuilt on the Hoe (a sort of park on the edge of town). It has been restored to original 18th cent. condition. It is now a tourist attraction. You can visit it and climb right to the top. This is probably the only rock lighthouse you can visit in the world...and a very historic one, being the first ever built.

If you want to combine several (or many) light houses, you may not be the best off with the country with the most Light houses but may want the stretch of coast where the most are easy to access.
If you look at a map of the Netherlands with the light houses on them, you will see that several are on islands with a few ferries each day. But the West coast of the Netherlands is almost just one straight line. On that coast are several easy to reach light houses.

Further inland are a few older ones, now mostly on the lake that was created when the inland sea was confined within a dike.

Down the coast, into Belgium you will find several more Lighthouses and into France some more.

Most of them, from Den Helder in the Netherlands to say Cherbourg in France, (or even farther south along the coast) are within one days driving. On top of that will come visiting time, but not a lot of time to get to remote places.

There are more coasts like that.
To visit many light houses I would go for the South coast of England rather than for the West coast of Scotland, even though there are likely more light houses in Scotland.

And for a very special night, he might want to sleep in a light house. This link leads to the Harlingen light house on the north coast of the Netherlands, mainland. When you search on staying in a light house you will find many more.