Hi @Monica, you should select a number of aCompCor components that explain a pre-specified percentage of variance, please check the corresponding section in the individual report generated. fMRIPrep organizes components by the variance they explain.
How you set that threshold is mostly based on experience. This paper - https://doi.org/10.1016/j.neuroimage.2017.03.020 should give you some of the recommendations you need.