No. In absolutely no circumstances should you ever send your ADA to anyone who claims to be running a staking pool. Delegating your stake to any pool does not require you to send your ADA. ADA will never leave your wallet. All delegation will occur within the Daedalus and Yorori Wallets.
In proof of work, miners invest computing power to compete to be chosen as the leader who gets to make the next block and win a reward for doing so. By contrast, in proof of stake, the stakeholder who will form the next block is randomly selected, proportionally to the size of the stake that they have, according to the blockchain ledger.
“Proof” means having evidence that blocks of transactions are legitimate. “Stake” means the relative value held by addresses on the node. “Relative value” is all the value held by wallets on a particular node divided by total value in the system.
There have been a number of attempts by other cryptocurrencies to develop a proof of stake algorithm, although these protocols have suffered from flaws and have not been shown to be provably secure.
For a blockchain to be secure, the means of selecting a stakeholder to make a block must be truly random. An innovation of Ouroboros to produce the randomness for the leader election process is to do this by way of a secure, multiparty implementation of a coin-flipping protocol.
No. Your ADA will not be locked or frozen for a certain time-period when delegated. You are free to move your ADA in and out of your delegated stake and free to change which staking pools you participate in as often as you want. The act of delegation does not relinquish spending power. Only the right to participate in the protocol is delegated. Funds can be spent normally at any time.
The Shelly Hardfork will occur on July 29, 2020, with the first rewards to be distributed on August 18, 2020. Stay tuned for updates.
There is no cap. However, there is an optimal amount of staking pools that the Cardano network should have. At this moment, that optimal number is 150 pools, and will change in a controlled manner as the network grows.