It depends what you mean by "sold a chip". Intel designs, fabricates and sells a wide range of chips, so for Intel the answer is simple (they don't license out their designs). But ARM doesn't manufacture anything physical. ARM produces the designs for microprocessors. Actual manufacturers such as Broadcom then license the designs, integrate them with other special-purpose hardware (e.g. a cellphone modem and keypad encoder) and then sell the result. The details are confidential, but basically ARM gets a commission on each chip sold. So how many chips does ARM sell? It depends on whether you count the chips produced by their licensees. If you only count chips produced directly by ARM then the number is zero. If you count their licensees then the number is massive.
In the 4th quarter of 2014 3.5 billion ARM-based chips were sold, which would mean around 14 billion per year. Meanwhile in Q3 of 2014 Intel sold "over 100 million" CPUs, which would mean over 400 million per year. I haven't found any total lifetime numbers for Intel processors, but if we guess that "over 100 million" in one quarter equates to 500 million per year, then it would take Intel 28 years to equal ARM's unit sales. Given the expansion of the PC market over the last few decades I doubt Intel was selling anything like 400 million processors in 1988, so the assertion is at least credible. Also note that this only counts Intel CPUs. Intel sells other types of chips as well.
Of course Intel processors cost a lot more than ARM processors. Intel specializes in being the fastest thing you can buy. ARM specializes in being the best choice for low-power battery operated devices designed to a price point. A typical Intel processor (just chip, packaging and heatsink) costs more than most of the gadgets built around ARM processors. Thats why the Raspberry Pi uses an ARM.