The modern answer: six
Since 1979–80, all forms of international cricket — Tests, ODIs and T20s — have used a SIX-ball over. After the bowler completes six legal deliveries, the umpire calls 'over', the fielding side switches ends, and a different bowler delivers the next six from the opposite end.
A 'legal' delivery is one that isn't called a wide or a no-ball. If the bowler oversteps the popping crease (a no-ball) or sprays the ball too far down the leg side (a wide), that delivery is re-bowled and doesn't count towards the six.
The history — it hasn't always been six
1880s — 4 balls
Test cricket originally used four-ball overs. The bowler had less time to settle into a spell.
1889 — 5 balls
England moved to five-ball overs to give bowlers more rhythm.
1900 — 6 balls
England standardised on six-ball overs, the format we know today.
1922–1979 — 8 balls
Australia, South Africa, New Zealand and Pakistan all used eight-ball overs at various points.
1979–80 — 6 universally
The ICC ratified six balls per over across all international cricket.
What doesn't count towards the six
A wide ball — the ball passes so wide of the stumps that the batter couldn't reasonably play a normal shot. One run is added to the batting side's score and the ball is re-bowled.
A no-ball — most commonly because the bowler's front foot lands beyond the popping crease. One run is added, the ball is re-bowled, and in limited-overs cricket the NEXT delivery becomes a 'free hit' where the batter can only be out run-out.
Can the same bowler bowl two overs in a row?
No. The laws of cricket forbid any bowler from delivering two consecutive overs. Captains must rotate bowlers from each end, which is why bowling partnerships are built around two players sharing the workload.
Frequently asked
How many overs in a T20?+
20 overs per side — 120 legal deliveries each.
How many overs in an ODI?+
50 overs per side, or 300 legal deliveries each.
How many overs in a Test?+
There is no limit per innings, but 90 overs per day are scheduled in a five-day Test.
Can a bowler bowl unlimited overs?+
In Tests, yes. In ODIs, a maximum of 10 per bowler. In T20Is, a maximum of 4.