How many transactions are there in one block What I understand:
When a transaction is made , miners compete to  validate it and create a hash . If one miner creates the hash and more than 51% of miners agree on the validity .Then the a block is added to the blockchain.
Am I right?
My doubt:
1)When a block is added it contains one transaction that is validated or more?
Thanks.
When a transaction is made, miners compete to validate it and create a hash.

Miners will compete to confirm the transaction in a block. 'Validation' of a transaction is completed by each and every full node on the network. Notice the difference between confirmation and validation:
Validation checks to see if the transaction is valid according to the network rules.
Confirmation involves having a valid transaction included in a block that is a part of the longest (most-work) chain.

If one miner creates the hash and more than 51% of miners agree on the validity.

As with transactions, a block is either valid, or invalid, and this fact does not depend on what other miners have decided. Full nodes perform this validity check, while miners just keep adding more blocks to the chain.

)When a block is added it contains one transaction that is validated or more?

Miners can choose whichever valid transactions they would like to include, up to the 'weight limit' of 4,000 kilo-weight-units. In practice, this generally means the block can be anywhere from a few hundred bytes, to 4 MB. These transactions are chosen prior to finding the valid hash, which means that once a valid block has been found, the transactional content of that block is locked in; the miner cannot add or remove transactions without invalidating the block.