Ans of course there is only one main blockchain.
I hope this reply can help you to understand a bit how it works.
It is "hosted" by many nodes in the bitcoin p2p network. Those nodes are full bitcoin nodes, meaning they hold a copy of the entire blockchain. The bitcoin blockchain contains all transactions of the network ever since Satoshi. New transactoins are included in blocks. Blocks are connected to eachother, hence the name blockchain. The blockchain is simply a large database. You can download the blockchain using Armory, bitcoind and other full bitcoin clients.
I am going to lock the thread, my questions are answered.