The slowest common 286 were running at 6 or 8 MHz (There was also a 4 MHz version, but I doubt that one was sold to end-users). The clock rate on Intel 286 chips was later increased to 10 or 12 MHz; clones from other manufacturers (AMD, Harris) reached 16, 20 and 25 MHz.

DOSBox cycle settings do not correspond to any MHz value, you can't compare them. Just keep on lowering them until speed feels right. I guess it would be less than 1000 cycles, though it varies with each program.

I am just wondering what clock rate should I choose for emulating PC AT with Intel 80-286 processor.
I've read that is was clocked at 20 MHz.

The Intel 80286 maxed out at 20MHz. Few AT class systems used so fast a processor. The IBM PC AT used a 6MHz 80286 in its first two iterations and an 8MHz 80286 in its final revision, all with one wait state. The IBM PC XT 286 used a 6MHz 80286, but had zero wait states for faster memory access. IBM PS/2s that used 80286s did so at 10MHz with one wait state.

My first IBM-compatible PC was an 8MHz Wyse 286. My cousins had a 10 or 12MHz 286.

Also, DOSBox cycles don't correspond to CPU clock cycles, as DOSBox does not emulate the CPU with that level of accuracy (since it doesn't seem to be required to do so in order to get a good level of compatability).

When there was only one processor, an 8088 @ 4.77MHz, many games were coded to run at that speed and would run too fast at a higher speed or fail to work at all. Once there were multiple processors to choose from, a programmer could no longer code a game to run perfectly only at one speed, he had to adjust for the range of processors available to the consumer at that time or ensure his game remained playable at the available speeds. This caused difficulty when the processors available to the consumer far outperformed the processors originally available to the coder.

I have an old Dos program called speed.com. It is very usefull to calibrate DosBox , because you can see the current speed based on ibm pc /xt(4.77mhz) and pc-at 286.
If you want , post here some way i can send to you.
EdusP

In my system , the CPU Cycles in dosBox to 1100 will be equivalent to 80286 running at 8 Mhz.
EdusP.