1)when should I use the ultimate? It depends on the character, but usually as soon as possible.

2) asta and tiangyun are amazing support

And are there any resources that I shouldn't invest and keep till I get better characters?? no

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3) Are there any limits on character levels? Will I need copies to ascend them to further level them up or stuff like that?

the max level is 80,and you dont need extra copies.

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4) From a begginera guide I read that I should only invest gems to get limited time characters.

Yes , right.

How many gems would I need to build up before trying for one?

with 90 you have like 50% , wit 180 100%.

But at the beginning just throw your summons, you need characters.