As dg99 pointed out, and from my own experience as well, there does not seem to be a limit to how many enemies you can tag at once. It would seem that it is only limited by the render distance of the game; sometimes human enemies glow a faint orange to signify their distance away, and occasionally I cannot tag them. Otherwise, you can tag all enemies that are within range of your Focus.
My favorite strategy that blends stealth with action is to create a mess of tripwires between you and your prey while utilizing Concentration and Tearblast arrows to stealthily take out opponents from afar.
From what I can tell, setting tripwires (though very loud) is perceived as silent by Machines. By that I mean you can place them literally on top of/through Machines and they won't have any idea who did it. Even when walking into tripwires, Machines typically will not find you if you're hiding behind rocks or in tall grass.
Once you've effectively set up a no-man's-land of crisscrossing tripwires in every direction (bear in mind that the maximum placeable at any time is 16), you can try getting headshots/destroying components of Machines. Another way to set these up is to basically fence a group of Machines by using tripwires. Electric will immobilize them and allow you to get a Critical Hit on however many were affected, while Blast wires will cause massive damage (and likely remove components).
If you are able to use them (they are on the Sharpshot bows), Tearblast arrows will do little to no damage, but will quickly remove the components of Machines (which effectively incapacitates them) and allow you to take them down more easily. Concentration will also help you hit these components and weak spots by slowing down time and zooming in, so it's very helpful for this.
If the Machine you're hunting notices you, they will most likely charge. At this point, they're extremely likely to run into at least one wire of your giant tripwire trap and then begin the process of stumbling into every other tripwire you've set. If this hasn't killed your prey yet, it should have come very close to doing so, at which point, you can use your weapon of choice to finish them off.
Best Answer
Since you are able to obtain spear mods, I can assume you have the Frozen Wilds DLC.
By default, you are not able to use any spear mods you come across. In order to unlock the ability to use spear mods, you must complete the quest A Secret Shared within the Frozen Wilds DLC area. It is one of the first set of quests available within the area, so it is pretty hard to miss.
Be warned though. The recommended level for this area is 30+, so you probably would want to power up before heading towards this area.
Aloy's Spear allows for one modification slot.
You can't purchase a new spear, but