There are 5 lagrangian points.
Are there any man-made satellites at any of those points? Is there a reason for the presence or absence of satellites at these points?
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First of all you need to specify the bodies you are considering.
There are 5 Lagrangian points in the Sun-Earth system and there are other 5 Lagrangian points in the Earth-Moon system.
Basically you can find 5 Lagrangian points (which are equilibrium points int the synodic reference frame) for each system made of 3 bodies where $m_3 << m_1,m_2$.
In the Sun-Earth system, L1 is preferred for Sun observation missions, whereas L2 is good for deep-space observation since it is possible to have the Sun always behind the spacecraft (which could be a telescope for instance) therefore it is possible to observe the whole emisphere without having the Sun in the field of view.
L4 and L5 are stable, which means that once you get there you do not have to spend propellant to stay there, whereas the others are not stable therefore you need to adjust your orbit nearly every period not to be driven away.
Missions in L1: Explorer 3, Genesis, Wind, LISA (coming soon).
Missions in L2: Herschel&Planck, James Webb Space Telescope (coming soon hopefully)
In the Earth-Moon system L2 is used for telecommunication satellites.