Here is the full list of all 18 Sonic Mania achievements. It takes around 10-12 hours to unlock all of the achievements on Windows.
Gotta gacha 'em all
Collect gold medallions in Blue Spheres Bonus stage
That elusive perfect run, only a professional can achieve
Collect silver medallions in Blue Spheres Bonus stage
You might have to submerge to find it
We really like boats
Get through Stardust Speedway Zone as quickly as possible
Let the wind take you through
Collect all seven Chaos Emeralds
Try for a 3 chain combo!
Have your photos taken in Studiopolis Zone
What would happen if you cross a bridge with a fire shield?
Achieve any ending
Barrel through the lava, don't let anything stop you
Try pushing a barrel to see how far it goes
Wreak havoc at the propaganda factory
Spin the Star Post!
Find the hidden item boxes at the end of the Zone