Now-a-days, these cloud computing users are more in many organizations. It provides many benefits are minimum cost and accessibilities of data. Ensuring the security of cloud computing is a major factor in its environment. Dealing with \"Single cloud\" providers is advance to become less popular with customers due to risks of services like availability failure and the possibility of harmful insiders in the single cloud. A movement towards \"Multi-clouds\" has emergency recently. This paper surveys recent research related to single and multi-cloud security and addresses possible solutions.