As M.S. assumed, I take it you are asking how many units of I you'd need to sell (at a discount of 86%) to equal or surpass selling just one item at 500.
Now, an 86% discount is a reduction in price in the amount of 86% of 500. Now 86% of a original price (we'll use p for price) is equal to \frac {86}{100} \times p = 0.86 \times p$. So you plan to reduce or discount the original price of 500 by (86% of 500 $= 0.86 \times 500) = $ $430.
So, to reduce the price from $500 by 86% means you need to subtract $0.86 \times 500 = $ $430 from $500 to obtain the discounted price of I: $500 - $430 = $70.
Now, in order to equal or surpass $500 (the gross yield from selling one I at $500), we want to know (how many I) x 70 dollars is greater than or equal to 500. We let $n$ be the quantity I (how many I), with each I selling at 70.
So we have the inequality: 70 \times n \geq 500$, and now we solve for $n$. Dividing both sides of the inequality by 70 gives us $n \geq \frac {500} {70} = \frac {50}{7} \approx 7.14.$
Now, it makes little sense to talk about .14 of an item (say a necklace), so we round up to $n = 8$, because $7 \times 70 = 490 < 500$, whereas $8 \times 70 = 560 > 500.
So, in this case, the answer is 8.
To generalize, if x$ is the number of I selling for 500 dollars, and $n$ is the number of discounted I you need to sell in order to meet or surpass sales of $x$ I's sold for 500 each, then you need to have $70 \times n \geq x \times 500$ that is, $ n \geq \frac {500x}{70} = \frac {50x}{7} \approx 7.14 x$
Then if you want know how many reduced-price I you need to sell to equal or surpass the cost of any number ($x$) sold at original price, you simply substitute that value for $x$ to obtain $n$. Note: if your result after multiplying x by $\frac {50}{7} is not a whole number, you need to "round up" to the next integer (whole number).
If you want to take into consideration the cost of producing and marketing each I (letting c = cost per item), then you need to work with the inequality: (70 - c)n \geq (500 - c)x$ or $n = \frac {(500-c)x}{(70 - c)}$
For this to mean anything, in terms of coming out with a defined and positive number $n$, the cost, c, of producing and marketing I must be such that $c < 70$.