The amount of people who have commented to you with completely incorrect reasons for why you only do compressions is something else. They’re all getting up voted, too.
But anyhow, for all of them: Single person cpr is non stop compressions at 100 to 120 bpm, non stop.
Two person cpr is the same rate, but two breaths every 30 compressions (2 every 15 for an infant).
Single person cpr is done that way now, because one person working by themselves can’t manage to effectively do both for very long at all before starting to screw up or move to slow, and the compressions are most important. It basically takes to long to stop, move up towards the head, tilt the head back, open the mouth, give two appropriate breaths while looking for chest rise, then reposition and go back to compressions.
Trust me when I say that you’ll be wore the heck out if you have to do 120 compressions a minute, almost hard enough to tear ribs from cartilage for more than a few minutes on your own. Adding in the breaths just isn’t something that has been shown to pay off.