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
You might have to submerge to find it
Wreak havoc at the propaganda factory
Let the wind take you through
Try pushing a barrel to see how far it goes
Collect all seven Chaos Emeralds
Find the hidden item boxes at the end of the Zone
Have your photos taken in Studiopolis Zone
Try for a 3 chain combo!
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
We really like boats
Spin the Star Post!
Collect gold medallions in Blue Spheres Bonus stage
Collect silver medallions in Blue Spheres Bonus stage
Get through Stardust Speedway Zone as quickly as possible
That elusive perfect run, only a professional can achieve