HS2 was originally planned to have a new station at Euston with 11 platforms. This was subsequently redesigned and reduced to 10 platforms. Last year the HS2 station at Euston was descoped and defunded, with the suggestion that the private sector would build a station with 6 platforms and commercial development on the site instead.
How many platforms will actually end up getting built? This will resolve when the rail is laid in the platforms, the site is sold for redevelopment into another use or HS2 is diverted to some other terminus (not Euston).
If there's a credible proposal for a new station at Euston with a different number of platforms, I'll split other to add that.
Looks like HS2 are still going to build the tunnel to Euston, whether or not there'll be a station at the end of it! One bit of evidence to suggest the number of platforms will be >0.
@alh More confirmation that the tunnels to Euston are getting built (FT article)