Why for every pair is required a keystone and a nic? Why is not possible to use all the needed pairs on the same keystone and nic?
Keystones are just passive connectors. You don't need them, just create the required connections any way you like:
What you need for 10BASE-T and 100BASE-TX is to split transmit/receive directions and connect each to a separate NIC's receiver. You need two NICs because there are two independent signals you need to receive (assuming full-duplex mode - with half duplex you just use a repeater hub and make sure that you're not sending anything).
In theory, you could try to use a managed switch in between the taps and a single NIC, but you'd need to set the switch to a single speed/duplex mode on the tap ports just like you'd need with two NICs. A single, static setting is required since Autonegotiation doesn't work with a one-way link. Each link has to go "up" on the detected carrier alone. However, if you've got a managed switch anyway, you could just as well use its port monitoring/mirroring function.
That mechanical splitting isn't possible for 1000BASE-T and faster at all since these use all pairs in full duplex - both directions simultaneously.
Actually, tapping into a gigabit link electrically is a rather complex endeavour as you'd need to separate the superimposed signals, which is much easier for the involved link partners than on an intermediate point - each partner knows what it's currently sending and can subtract that from the received signal (greatly simplified).
Instead, you'd need to terminate the link in between and tap into the terminated link. The most common method is probably a managed switch with port mirroring set up.
