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no rules on steemit but guidelines you should try not to upvote to much per day because each time you upvote your voting power goes down meaning your vote is worth less and then takes time to get back to 100% I try to not vote more than 10 things per day but it's up to you what post you want to vote on.

Even when voting comments the voting power goes down?

yes you can learn more in the FAQ section.

also to learn more you can check the Steemit guidelines FAQ page https://steemit.com/faq.html you can type hold down ctrl+F to bring up the search bar and search the word upvote then you can click the down arrow to easily skip to each section about upvotes to learn more.

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