depends on game mode and opponent.

Generalizing will generally use to a focused army though

neither, a middle ground is best. if you heavy invest into one unit, then you leave yourself open to having that unit countered and then you lose. if you do the opposite youll have a bunch of units that dont really add much to your board and could end up losing to the previous (a hyper teched) unit.

Okay so 1 vote more towards teched unit and another one for a mix. Well my vote as you understood is in favor of more unit variety with less upgrades.
Also for the fact that yeah, you get a lot less easily hard-countered and you can yourself counter more easily by adding just the one unit type you need with no or 1 upgrade.

Also towards the late game it's more easy to add a second upgrade to 1 unit type rather than a third one that is going to cost me like 900 on a unit I already have.

Now to go in the sense of Dominic I am thinking of a middle ground strategy about having just one unit type that you will tech heavily (carry) and the rest would be very diversified and with almost no upgrades.

Everything depends on the board.
Both variants can be good.
Be adaptive.

Some people swear on focusing only on 2 units and selling others away, only adding a counter when absolutely needed. And it works for come combos for them.

I personally like to try all units out and have many unit types. Still usually have 1 major unit though.

In general you should only get new unit types when they serve a purpose that an upgrade cannot.

Iam playing in 1700-1800 MMR. I usually find myself having 1-2 Tech's per unit type unlocked in the end of a Match.
I very rarely use 3 Tech's on one unit if I clearly see my Opponent isnt clearly reacting to me properly or I have a very hard counter allready and especially by doing so - Considering there is only a low chance to easily counter my 3 times Teched up Unit by opponent side.

How many unit types? I just checking which weakspots I need to close: Is my
Chaff,
my Anti Swarm;
Single Target;
Anti Air - Single Target,
Anti Air Swarm
- enough or not.

If a unit type doesnt make any sense any more (another unit from my side outperforms another unit that deals with the same job (Wraith versus Arclight)) I simply slowly replace it with another unit and until some critical point reached I also start to sell unefficient units.
For example I can sell all Arclights if I have a critical Mass of better Anti Chaff units; Or selling all Farseers if there is enough Anti Air and High Damage units on my side, since he is quite slow with firerate and the money inside him could be better used maybe; or selling all stormcallers once I have more efficient Area Damage units on the ground because their firerate is slow too.
It depends on the situation, sometimes a stormcaller is usefull to keep to have it as a Tech option as a support (EMS or Fire). But thats another story.

One of each unit with as many techs as possible is the best way.