Probably less then 2-3%. If we look at some army sizes for England:
In the late 13th century Edward I expanded the familia regis to become
a small standing army, forming the core of much larger armies up to
28,700 strong, largely comprising foot soldiers, for campaigns in
Scotland and France. By the time of Edward III, armies were
smaller in size, but the troops were typically better equipped and
so we had an armed force of, say 30,000 max, probably smaller. Population of England at that time was about 2.5 million, so 30,000 / 2,500,000 gives about 1.2%.
That's just a rough guesstimate at one moment. Just a few decades earlier the population was much larger, so the percentage would have been accordingly less.
How many individuals had amour but weren't in someones army? Its still minimal but that's why I allow double the size of the largest force I find mentioned.
If you find more precise data just reference the population page above to get numbers concerning the relevant percentage.