If I have an Ultrarare Character at VI...how many points am I missing compared to X?

How many IRL $$$ have you spent on microtransactions to get a UR to VI?

You get 4 extra skill points every other rank (rank II, IV etc) for a maximum of 20 extra skill points. So at rank VI you have 12 and will get another 8 by rank X.

Once you max out your character to level 20 and rank X they will have 85 skill points allowing for a 6/6/6/6/1 build or whatever variant on that you like.

I think it is a little bit more, or I am misunderstanding something. My level 1 / rank I toons have 2 spared skill points. My level 1 / rank X toons have 25 spared skill points. The bonus skill points yielded by even cards should thus be 23 unless I miss something. It should also not be evenly distributed across the ranks, for obvious reasons. But as I said, I might miss something.

You are right sorry, what I wrote is not quite correct. I think the following is actually how it works:

One skill point is put into each power as default(so three). Then you are given two additional skill points at level one to do with what you wish. So a rank I level 1 characters starts with 5 skill points. Then you receive 3 skill points every level so a rank I level 20 character has 62 skill points.

Rank II and Rank IV give an 4 additional skill points each.

Ranks VI, VIII and X give an additional 5 skill points each.

Therefore total additional skill points are 23, added to the 62 gives 85.

So technically at rank I you can only max two skills making the ranks very important.

Hopefully I have got it just about correct this time.

It's just what I've managed to extrapolate from looking at my own characters. But hopefully it's reasonably correct.

Can you do quick math for what's the max possible build options for grade I characters?

All of them.