a better term to be used instead of confirmation that would also remove the confusion you have is "depth". when you say a transaction has 260000 confirmation you are really saying that this transaction is 260000 blocks deep. or in other words there were 260000-1 blocks found after the block that included that transaction.
so no there is no maximum limit for that number, as long as bitcoin exists and its blockchain is growing that number is increasing for any transaction.
Every depth block will cost computation power, am I right?
If the amount of transaction is very small, and the depth is infinite, will it become a burden to whole bitcoin system?
It is really just the age of the transaction --the number of blocks since the transaction was included in the block chain.