As the TE is increased:

Increasing the echo time (TE) in MRI has a direct impact on the relaxation characteristics of the tissues being imaged. TE is the time between the application of the RF pulse and the signal readout. As TE increases, the time allowed for T2 relaxation also increases, which leads to a greater decay of the signal from tissues with shorter T2 times.

When TE is lengthened, the overall time of the imaging sequence is extended, which can lead to a longer repetition time (TR) if the sequence timing is adjusted accordingly. A longer TR may reduce the number of slices that can be acquired in a single scan since the imaging sequence takes longer to complete. This is particularly true when you are trying to acquire multiple slices in a multi-slice sequence.

Consequently, while long TE emphasizes T2 contrast in images, it often does so at the expense of the total number of slices you can effectively collect within a given time frame. Thus, increasing TE typically results in a decrease in the available number of slices that can be acquired.