It sounds like you have a few fundamental things you're getting confused about with contracts and ether: Firstly, you as a human (or company) owns an Ethereum address which you can use to transfer ether into and out of (your wallet). When you deploy a contract, that action is a transaction in the Ethereum blockchain, and so has to come from some Ethereum address. So, you sign it with your wallet address, and the Ethereum virtual machine creates a brand new address (separate from your own wallet address) for the new contract.
Anyone can send ether to a contract. But getting ether back out is dependent on the code of the contract itself. You need to pick how that's going to work as you develop it.
So, to answer your questions:
The gas [to deploy the contract] will be extracted from the ether that the account has right?
The account that's deploying the contract (your wallet) must have enough ether in it to pay for the gas to create the contract, yes. The newly-created contract will be created with no ether in it (unless the code of the contract does something that transfers ether in).
The gas is said to be consistent but the number of transaction cannot be predicted. It can be few or huge so how we calculate the ether we need in first place?
The amount of gas needed to create a new Contract depends on the code complexity of the Contract itself. You can use the
estimateGas() function in web3 to calculate that from the compiled bytecode of your contract before you deploy it (more details).
How you should set up the ether amount and make sure later it not runs out
Both your own personal wallet and any deployed contract can receive ether paid to it. So, if your deployed contract needs to send ether to someone, and it runs out, you can send more to it at a later time. However, it's possible that a contract, once deployed, will not ever need any ether, since it doesn't pay for the gas needed to trigger its methods. A transaction needs to be crafted to call functions on that contract, and that calling function has to come from a human-owned contract, and the human-owned contract pays for the gas of the transaction.