How many solo concerts has Fat Tiger held, and were they successful?

Ha, you've hit the nail on the head! As a classic recurring gag in Doraemon, Gian's concerts are one of the core "disasters" woven throughout the entire story.

Let me break it down for you.

How many concerts has Gian actually held?

An impossible-to-pin-down number, essentially "countless."

Why is that? Because "Gian holds a concert" is a classic, recurring comedic trope (running gag) in Doraemon, much like Nobita getting zero on a test or being chased by a dog. It appears in the manga, both the classic and new anime series, and the movies.

No Official Record: The official creators have never, and could never, tally an exact number. It's simply a plot device.
Varied Formats: Sometimes it's him singing atop a concrete pipe in an empty lot, gathering a few friends. Other times, it's in his own backyard, and he's even held seemingly grand events like the "Gian Super Concert."
Impromptu Performances: Often, Gian just gets the urge to sing on the spot, grabbing Nobita and Suneo to be his "audience." These might not even count as formal "concerts," but the effect on the listeners is identical.

So, just know that Gian's passion for music is unquenchable. Whenever he feels like singing, anywhere instantly becomes his stage. The precise number isn't important; what matters is his sheer dedication.

So, were his concerts successful?

This question has two angles, with completely different answers.

1. From Gian's Perspective: A Huge Success!

In Gian's eyes, every single one of his concerts is an audio-visual feast, a perfect artistic presentation.

Massive Ego Boost: He's utterly immersed in his own singing, convinced he's a superstar with a voice so moving it's unmatched.
"Enthusiastic" Audience: He sees the "seats" filled (even though everyone was threatened into attending) and believes his music has immense drawing power.
Full Commitment: He pours his heart and soul into every performance, singing with total emotional intensity. By the measure of effort, it's 100% successful.

For him, if he enjoyed singing, the concert was a success.

2. From the Audience & Objective Perspective: An Epic Disaster!

This is the common understanding for us readers/viewers and the source of the humor.

Listener Torture: Gian's singing is described as "ear-splitting," possessing potent "physical and mental attack power." Listeners (mainly Nobita, Suneo, Shizuka, etc.) experience dizziness, nausea, and a desperate urge to flee – it's pure torment.
Mandatory Attendance: Concert "tickets" are usually free, but attendance is compulsory. The audience is "invited" via Gian's fists; anyone who tries to run gets beaten. It's less a concert, more a "suffering session."
Destructive Power: His voice can shatter glass, drive out rats and cockroaches en masse, and even cause nearby people and animals to faint. In some movies, his singing is even used as a "weapon of mass destruction" against enemies.
Commercial Failure: Occasionally, he holds "paid" concerts, forcing the audience to buy terrible snacks he made himself as tickets. This business model is only viable in his own mind.

Summary

So, in short:

Gian has held countless solo concerts. By his own standards, each one is a resounding success. But by everyone else's standards, each one is a complete and utter, terrifying disaster.

And this massive contrast is precisely one of the most endearing and hilarious things about Gian's character.