During a discussion with a colleague, I came to know that for motors with antifriction bearings, you only need one accelerometer to measure the vibrations, whereas for motors with sleeve bearings two accelerometers are needed (one for measurement in X direction and other for measurement in Y direction).
ALL bearings are "anti-friction", otherwise they would not be bearings.
There are basically two types of bearing: non-contact (e.g. magnetic) and contact. The contact type can be further broken down into sleeve (or journal) and rolling element (which you're incorrectly referring to as "anti-friction").
Rolling element bearings can use multiple probes (or single probes with multiple capabilities), but sometimes only a single direction is measured. The direction can be either X (horizontal) or Y (vertical). If an axial (Z direction) measurement is required, a separate probe is usually necessary.
Sleeve bearings only touch a portion of the shaft; this means the shaft is free to shift in multiple vectors relative to the bearing surface (which is not the case for rolling element designs).
The choice to use a single probe on rolling element boils down to a couple of ideas. 1.) A single probe (for one direction) is cheaper than either a multi-vector probe or a multi-probe solution. 2.) Since the bearing is in consistent contact with the entire shaft surface, what is measured in one direction applies to all directions (a fallacy). 3.) The size and style of the machine construction - including the bearing support mechanism. For rolling element bearings, the structure is fairly "stiff" in both horizontal and vertical planes. This means only the direction of probable imposed thrust needs to be considered - which may be related to the foundation stiffness in each direction.
For sleeve bearings - since the shaft can move relative to the bearing surface, multiple vectors need to be considered to accurately determine what is happening.
Best option for all bearings regardless of type: two multi-vector probes at roughly 45 degrees to horizontal plane.