Think of 2-3 shrimp per taco and then decide how many tacos you'd eat and serve per person. Buy shrimp according to that and scale the rest of the meal up or down based on those calculations. You can buy shelled and cleaned shrimp to make life easier, but if you want the cheaper route just simply remove the shell and devein before marinating.
1) Marinate the shrimp in a splash of olive oil and a large handful of finely chopped cilantro (including the stalks) for 1/2 an hour in the fridge.
1b) While the shrimp is marinating thinly slice the cabbage and carrots and toss with a handful of roughly chopped cilantro, a couple spoonfuls of mayo, zest of half a lime, and juice of a whole lime (or add more, depending on the lime's juiciness).
2) Heat a skillet or griddle over moderate heat and cook the shrimp 2-3 minutes per side until pink and firm. Warm tortillas and build the tacos with slaw, shrimp your fave hot sauce, sour cream, avocado, cilantro, and another squeeze of lime.