Yes you can put just one item in the washing machine if you want too or need to.
Just place the water setting on the lowest setting when washing one article of clothing or one sheet, towel etc.
It's more energy efficient to wash more than one item or article of clothing in the washing machine at one time but you can wash just one article of clothing or one item that you have if you want too.
There have been times where I had to wash a pair of pants or shirt by itself but mostly I wash a full load or at least a half load of clothing at one time.
Because that saves me on my electric bill and water bill as well as detergent.
But if you need to wash something then do so as it will do no harm to the washing machine.