do you think lemmy will ever have millions of users

It's not infeasible. It won't likely happen tomorrow, and to manage it will take massive server resources or many more instances, but as disillusionment grows with corporate social media I hope to see more people here eventually.

and niche topics or subs like reddit?

There's plenty of niche communities already across the fediverse, and I'm sure they'd love whatever you'd like to contribute to them.

or even be a top result when you search google? like reddit posts

Possibly, but probably not. The incentive structure for something like Google is built around profit, and the fediverse isn't really built to profit, mostly just financially tread water at best.

If enough people join then Google would be shooting themselves in the foot financially to not return lemmy in its searches if for nothing else than its own relevance, but as a general rule they'll follow the craven flow of capital every time.