[sgdpk]

So, for your first questions, you can definitely entangle as little or as many qubits as you want. In practice, it gets harder the more qubits you want to entangle. But a state like the GHZ state can entangle all qubits in your system.

How do you entangle? You can just let two particles interact, so they "mix" their information. Example: you send laser light (photon qubits) to an atom (another qubits). After a short period, there is a probability that the atom absorbed photons, but that probability is not 100%. You just entangled light with an atom. Only measurement can give you information on whether the photons were absorbed or not.

In practice, each platform has its own entangling mechanism. Usually, it entangles only 2 qubits. Many-qubit entanglement can be achieved by pairwise entangling AB, BC, CD, etc.

The first practical example of qubit entangling operation was the Cirac-Zoller gate, you can check it out.

Regarding your second question, you can actually measure just part of the system, and measure the rest later. It will give you a partial collapse. It's called "tracing out", the quantum analogue of marginalization in probability.