As with any time these questions come up, the stocking rate your land will sustain will depend heavily on quality and quantity of forage, rotational intensity, how fast your forage regrows, size of cattle, and other husbandry factors. Where one person can run 1 cow per acre, others need many acres per cow. Obviously bringing in outside feed (whether hay or grain) will reduce the acreage required. So unfortunately there's no really effective "rule of thumb" for stocking rates.
If it was me, I'd start with 2 cow/calf pairs and see how it goes. If you're having to mow to keep the pastures from getting out of hand, add more cow/calf pairs. If you're needing to bring in hay for a few months of the year, back it off by a pair. Most likely, knowing where you are in the growth patterns for the pastures, and seeing how much 2 pairs are eating, you'll get a good handle on whether you can add more within a month or two. If you do start adding pairs, do it conservatively. Better to be a little understocked than overstocked if you're wanting sustainability and minimal to no outside inputs. IOW, if you quickly realize you can add more cattle, and you think
6 pairs would be fine, go to 4 or 5 and see how that goes for a while before going all the way to 6.