All stream items go into the same blockchain. A stream item is just a special type of transaction which is tagged as containing data. Nodes which subscribe to the stream then index that data in real-time.
The limit on stream items per block depends on general transaction limits – see 'Production Recommendations' at the bottom of this page: https://www.multichain.com/developers/blockchain-parameters/
There is no meaningful difference in performance between using one or multiple streams. But you might want to consider using off-chain data (which has built-in support in MultiChain 2.0) for greater flexibility.