Final answer:
Amino-acyl-tRNA synthetases charge tRNAs with amino acids and there is at least one for each of the 20 amino acids, so there are at least 20 of these enzymes in a given cell.
Step-by-step explanation:
Amino-acyl-tRNA synthetases are essential enzymes that play a pivotal role in the process of protein synthesis. These enzymes are responsible for charging tRNAs with their respective amino acids, which is a critical step in translating the genetic code into functional proteins. There is at least one type of aminoacyl tRNA synthetase for each of the 20 standard amino acids found in proteins. Therefore, in a given cell, there are at least 20 aminoacyl-tRNA synthetases.