How many transactions could be included in the next new block from the same account? From my understanding, every transaction would have a unique nonce. This nonce of an account would be increased by 1 for every transaction? So I wonder if it can just have at most 1 transaction from the same account.
Multiple transactions/block is fine. Throughput is ultimately limited by the block gasLimit. More on block gasLimit here: https://bitcoin.stackexchange.com/questions/39132/what-is-gas-limit-in-ethereum
Additionally, blocks, too, have a field called gas limit. It defines the maximum amount of gas all transactions in the whole block combined are allowed to consume. Similar to the maximum block size in Bitcoin (measured in bytes), its purpose is to keep block propagation and processing time low, thereby allowing for a sufficiently decentralized network. In contrast to Bitcoin, it is however not a constant. Instead, miners have the option to increase or decrease it every block by a certain factor. 
 See the Yellow Paper for a breakdown of operations and the respective gas costs (Appendices G and H)  https://github.com/ethereum/wiki/wiki/Design-Rationale#gas-and-fees  See the Yellow Paper Equations 40 to 42 for the exact rules