You only need 150 strawberries to achieve the best ending. Collecting all 200 strawberries will add a golden strawberry stamp to your save file, but it will not change the ending.
The video above by Git Gud shows that there are 5 different endings for Celeste. I have transcribed each ending below with the number of strawberries needed and a screenshot of the final pie.
Ending 1 (0-29 strawberries)
Theo: I guess this is technically a pie...
Mr. Oshiro: Is this some kind of joke, Ms. Madeline?
Old Woman: Strawberry pies are supposed to have strawberries in them, dear.
Ending 2 (30-49 strawberries)
Theo: Madeline, I super appreciate you baking for us... But I don't think I can eat this.
Old Woman: It's the thought that counts.
Badeline: You people just can't appreciate good food. I'll eat all of it myself.
Madeline: You don't have to do that.
Badeline: Yeah... it was an empty threat.
Ending 3 (50-89 strawberries)
Mr. Oshiro: The important thing is that you tried your best, Ms. Madeline.
Badeline: Who invited this guy?
Theo: Come on, it's really good.
Old Woman: I've certainly had worse.
Theo: Thanks for the pie, Madeline.
Ending 4 (90-149 strawberries)
Old Woman: I'm impressed! This gives my baking a run for its money.
Badeline: Not bad.
Theo: Holy crap! This is so tasty!
Mr. Oshiro: Ms. Madeline, are you available to cater our re-opening banquet?
Theo: Can I take a slice home for Alex? She'll die when she tastes this.
Ending 5 (150-200 strawberries)
Theo: Madeline. This is the best thing I've ever tasted!
Madeline: Come on. Be honest.
Old Woman: He's right, dear. This is delicious.
Mr. Oshiro: It's truly magnificent, Ms. Madeline. A symphony for the taste buds!
Badeline: Can I take partial credit for this?
Madeline: Wow, thanks everyone. I worked really hard for it.