Top lane can generally buy wriggles for both lane sustain and free wards as well ^^

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1 at a time for AP and AD carries.
1-2 at a time for solo top.
2+ at a time, or one and
2-5 at a time for supports.
Not a rule, but that's how I usually buy wards. Carries do need to save up their gold, but you absolutely should still get wards, especially if the rest of your team is neglecting them.
With the Chaox "Wriggles. On. EVERYONE" build, you really don't need to buy any wards as an AD carry (provided you get wriggles). This is because you're basically only warding to make sure you don't get ganked while farming, and wriggles provides that 1 ward whenever you need it.