Is there any sequence that has infinitely many cluster points?If there is,what are the sequences that have this property?
It is a concrete example!!! $\displaystyle \mathbb{Q}$ is countable and dense. Countable means that there exists a bijective aplication $\displaystyle \pi:\mathbb{N}\to \mathbb{Q}$. Hence $\displaystyle (\pi(n))_n$ is a sequence containing ALL the rational numbers (not formed by rational numbers). Since $\displaystyle \mathbb{Q}$ is dense in $\displaystyle \mathbb{R}$, every real point is a cluster point of this sequence.
If you want a simple constructive formula then you could take $\displaystyle x_n = \sin(\ln n)$, which oscillates slowly between –1 and +1, so that every point of the interval [–1,1] is a cluster point. That also provides an answer to the question in your other long-running thread Limit of a Sequence.