How many transmitters do you need for a 5 piece band?
For example here: http://www.thomann.de/gb/ld_systems_mei_1000_790_mhz.htm
there is 1 transmitter and 1 receiver, obviously you need 5 receivers and 5 earphones for each band member, but how about transmitter, is 1 enough?

What is the most important thing in in-ear-monitoring: earphones, receivers, transimitters, etc? Also, is that example system any good or maybe you could suggest something else?
It depends on what each band member wants to hear through the monitors and the number of send/returns you have available from the monitor desk at a gig.

Some guitarists like to have some of the drums and bass only coming through (kick, snare and the bass).
In theory that unit should be capable of transmitting to each person in a five piece, but you would each receive the same mix, as far as I can tell - although I'm not a monitor tech.

Which leads me to the (lame) joke: What's the difference between a monitor tech and a toilet seat?

A toilet seat only has to deal with one a$$hole at a time.
It's an opinion. It's subjective. And I'm right, anyway.