How many blocks needed to be confirmed in Arbitrum?

It depends on which level of finality you need please check the transaction lifecycle Soft confirmation from the sequencer is instant, a stronger confirmation is when the tx is included in a batch posted to L1, which is around 3 minutes but you need to query this function in the NodeInterface instead of wait for a set number of blocks https://github.com/OffchainLabs/nitro/blob/e10b61e12105be459a1c8df1d66f0db7ec289c58/contracts/src/node-interface/NodeInterface.sol#L71