1) We have 17 leaves, 17x3=51 characters of WIF. WIF is encoded in Base58 (26 letters capital and small and numbers without O, 0, l, I, +, / - so it's smaller Base64).
Could anyone tell me what's the lowest bit number to encode it ? Is 6 bits enough or we need at least 7 ?
Yes, 6 bits is sufficient to cover Base58. But if you come across 6 bits that represent a value >= 58, you have to decide how to handle that as it would not address a B58 character. You can wrap around or consider it invalid.