HBO confirmed a while ago that while they had multiple concepts (5 according to GRRM), they only planned to have one show.
"We had four scripts in development when I arrived in LA last week, but by the time I left we had five," George RR Martin said in a blog post.
But HBO's President of Programming Casey Bloys then took it upon himself to clarify what was actually happening, announcing that while several shows are in development, HBO only expects to produce one in the end: "The idea is not to do four shows [or five, as we now know]."
https://www.digitalspy.com/tv/ustv/a835085/game-of-thrones-prequel-spin-off-show-cast-date-ideas/