All skills CAN be maxed HOWEVER there are around 280 perks, and around 70 levels. A perk per level means you will only be able to have about a quarter or a third of the skills per playthrough.
There are 18 different skills which can all be level 100/100 however only around 6 skills can be fully complete with all skills and perks. This means you can have all FULLY maxed skills in 3 playthroughs
however there are many different combinations of skills with only 6 being maxable per char so there are many combinations to make unique charecters.
^^ This works for me so. If you can have 6 FULLY MAXED skills (This meaning with lvl 100 and all corresponding perks for the skill) what will you chose?
Only pick 6 skills!