I realize this is subjective, but how many shards could you lose before you just no longer have much of a chance of winning anymore?

I was just wondering, I lost my first shard battle because I had no idea what I was doing, I haven't lost one since, but it is a little disturbing that if you play poorly you could lose the campaign entirely and have to start over. That is a huge time investment. So how many shards could you lose before you just no longer have much of a chance of winning anymore?

Based on difficulty, but even 15 is not a serious problem.
On beginner max is around 50.

That is a lot more than I expected. I was guessing (based on nothing) five or ten on beginner. Good news.

Hi there, another question regarding losing shards. If i lose a shard, are the buildings of that shard gone forever or do they simply spawn on another one?

Lost my first shard on "competent" because i though: "why not choose a commander as main hero this time?". Big mistake.

They simply spawn on another one. Otherwise it would be impossible to get them all, most of them are prerequisites.

Thanks for your fast answer!

Which brings up the next question. How many shards do you have to win in order to win the campaign?

It depends on which ending you're going for. I think the shortest path is the Keys of Truth ending, which could be done in about 30 turns.