The number of backlinks needed depends on the time available to build them. Backlinks, links to your website from another website, should always be thought of in terms of quality, rather than quantity. Never buy links to your website and never post links on websites that seem sketchy. One quality backlink from an authoritative source will provide better results than multiple less reliable sources. Websites with a .gov or .edu domain provide even more authority as those sites have stricter requirements in obtaining them.