Is it safer for a country to have many different allies or to rely on one very powerful relationship?

NATO is based on collective defense, this means that if a country is attacked, all others will help. So EU countries aren't only protected by the USA but also by each others. This created a strong security if a country only relies on the USA change its foreigner policy on reduce its support, that country could feel less protected. In my opinion it is safer to have a lot of allies like NATO. If a country only relies like the USA, then it can become too dependant. With severals allies, countries have more supports and more security.