4 EV's are equal to 1 stat point, so that mean 252 EV's will add 63 points to a stat, maxing it out at level 100. However, since in VGC (and all wifi battles) all pokemon are reverted to level 50, the conversion for EV's to stats becomes 4 EV's for the first stat point, 8 for the rest, adding 32 points to a stat if maxed out. Due to being limited to only 510 EV's total, you can only max out two stats, leaving 6 EV's leftover for another stat.
That's not always the best way to go though. Sometimes it is better to cut back on a stat to the bare essentials so that more EV's can be allocated to other stats. For example, let's say you're setting up a scarf-Hydreigon. First you'll want to start with the speed stat. Looking at this VGC Speed tiers spreadsheet
set up by the good folks at Nugget Bridge, we see that the fastest un-boosted mons usually top out at a 200 speed stat, so a speed of 201 is what the mark we'll try to hit. Multiply 201 by 2/3 to compensate for the scarf and you end up with 134 as the target speed stat. It can also be bumped up to 135, as joej m did with his Landorus-T, allowing you to get the jump on other scarfed mon's running at 134. Assuming the 134 speed stat though, that'll will only cost a Modest Hydriegon 124 Speed EV's, leaving 128 EV's leftover for other stats. For the rest of the stat's you'll want to refer to a good damage calculator. Naturally, you'd want want to max out Hydreigon's Sp. Atk, so we're left with 134 EV's total to work with. Now you could pour all those into Hp to add to Hydriegon's bulk:
252+ Atk Technician Breloom Mach Punch vs. 134 HP / 0 Def Hydreigon: 126-150 (68.47 - 81.52%) -- guaranteed 2HKO
But... if you take out 8 Hp EV's and divvy out 4 to both Def. and Sp. Def.:
252+ Atk Technician Breloom Mach Punch vs. 126 HP / 4 Def Hydreigon: 122-146 (66.66 - 79.78%) -- guaranteed 2HKO
You get a slightly bulkier set. Ultimately you want to optimize your EV spreads so that each pokemon can perform it's role on the team to the best of it's abilities.