JACE debuted as a 4 member band under Right Achievement Group in 2014, but are currently a duo.
After three of the members - Youngsup (leader and drums), Dohyun (guitar and vocal) and Sangeun (bass and vocal) - left, only Seunghyun (vocal and guitar) stayed behind. The company then added a new member - Junghwan (guitar) - but they haven't made a comeback in this line-up yet.
I don't know for sure when the three members left. I can't find any live performances Radio Star, so I can't confirm if the other members are still on stage. And the album cover and mv only had Seunghyun in them.
Yet the comment section of the video for Radio Star doesn't contain any messages that indicate the other members had already left and the label still tagged JACE as a band instead of as a solo act what JACE would've been then.

Cover with the current line-up