There are 50 Trophies that can be collected.

Bronze

The Opening Act
Race through the first stage.

Greased Lightning
Clear GREEN HILL Act 1 within one minute.

Bright Star
Get Rank S in an Act.

Shooting Star
Get Rank S in three Acts.

Trickstar
Pull off a seven or more trick combo or six trick combo ending in a finishing trick.

Eradicator
Defeat 100 enemies.

Ring King
Reach the goal without dropping any of the rings you collected in GREEN HILL Act 1.

Action Hero
Perform all of Sonic's moves in Act 2.

Bonds of Friendship
Complete all Challenge Acts featuring Sonic's friends.

Walkie Talkie
Chat with each of Sonic's friends you have saved.

Join the Ranks
Join the rankings.

A 30-Second Test
Participate in a 30 Second Trial.

Silver

All Stages Cleared!
Clear Sonic Generations.

Blazing Meteor
Get Rank S in seven Acts.

Blue Comet
Get Rank S in twelve Acts.

Time Attacker
Play Ranking Attack on all stages.

Red Ring Collector
Get all Red Star Rings.

Halfway Point
Clear half the Challenge Acts.

Mission Accomplished!
Clear all the Challenge Acts.

Gold

Big Bang
Get Rank S in all Acts.

(Hedge)Hogging It All Up!
Get all collectibles.

Platinum

All Trophies Collected!
Collect all Trophies.

28 Secret

http://www.ps3trophies.com/forums/ps3-trophy-news/4565-sonic-generations-trophy-list.html