I'm trying to work out what to spend my points on for my multiplayer character. How many skill points will I have in multiplayer at level 20?
You will have 84 skill points at level 20 of Mass Effect 3, if none are spent already.
At level 1 you have only 1 skill point, at level 12 you have 40 skill points, at level 16 you have 60 skill points and at level 20 you have indeed 84 skill points.
Possible distributions for the 84 points for your 5 skills are: 6-6-6-6-0 or 6-6-6-5-3 or 6-6-6-4-4 (1 point unused) or 6-6-5-5-4 (2 points unused) or 6-5-5-5-5 (3 points unused). Points unused does not necessarily mean that that character build is inefficient. Depending on your race and character class and your play style, it can be a very good skill distribution.
There is also a website containing a good online calculator to simulate different skill upgrades for most character classes (no resurgence characters so far unfortunately).