Lied -liederen Liedje - liedjes
Liedjes is the popular term for songs. Liederen would be used for things like psalms or more serious music, when you use it in its plural form. You are more likely to say opera liederen and operette liedjes for example. Lied is neutral. Lied and liedje are used far more often than the word liederen.
I tried "How many songs will they play", and it wasn't accepted. As far as I'm concerned "Hoeveel lidjes zou ik spelen" means "How many songs will i play" (although I know that sounds a bit weird).
Why is "would they play" accepted and not "will they play"?
- How many songs would they play? - Hoeveel liedjes zouden zij spelen?
- How many songs will they play? - Hoeveel liedjes zullen zij spelen?
The tense is different.
As a correct answer "How many songs would they play?" seems fine to me as the question would refer to how many songs would they be able to play in their time slot and then other bands would have also a time slot allocated and so they would be concerned as to how many songs they could be played in that time.