What could be the issues with my 2001 Ford Taurus when I turn the key, all you hear is one click sound. Sometimes it starts right away and sometimes it starts after 20.

Sounds like your starter is about to go or you may have a loose connection at the starter have wires to starter checked and tighten.

They had trouble there at the starter connection, the wire may show power with a test light every time, but sometimes the connection can be intermittant, many have been found to have dirt and oily greasy between. We were upgrading the nuts and washers to increase surface area contact, if that was where testing normally led us. start at the battery, clean and tight, go component by component starting with the starter working your way back to the battery after that, but it has to be acting up. You can remove and bench test the starter rather than guessing. as well.