Why did so many join him eastwards?

Because he was a powerful Jarl and nobleman, who most likely had direct ties to the royal family. Naturally, ambitious warriors were flocking to him.

Why was he called the far-travelled (he was only 25 when he died so he couldn't have travelled that much)?

No, not much, but far. They travelled from Sweden in the Baltic, to Georgia by the Caspian Sea, which is an insane distance with the technology of the time.

How many would have joined him on his trip and why are there so many runestones describing it compared to other viking raids?

We don't know how many. A medieval Georgian chronicle, thought to be referring to Ingvar and crew, claims there were 3000 of them, which is hardly believable. A couple of hundred at the most, probably. The amount runestones is most likely because of the scope of the endeavour. This was pretty much Swedens answer to Harald Hardradas invasion of England. One last massive adventure, before the Viking Age ground to a halt. And since it likewise ended in disaster, and considering Ingvars noble background, it most likely had serious consequences for Sweden as a whole.