(Defeated Pokemon EVs + Power item 4) x2
If you defeat a Zigzagoon which gives 1EV in speed and add 4EVs from the Power Items. In total it gives you 5. You double 5 to get 10EVs.
Just after 25 Zigzagoons you'll get 250 EVS. So one more Zigzagoon will get you to 252.
In total after 26 Zigzagoons you will max out your Speed Stats