A sales rank of #5,600 sounds good, but how many is that a month? Rank alone can't tell you: #5,600 in Grocery sells far more than #5,600 in a small category, and rank moves every hour. Two better signals turn rank into sales.
What sales rank is
Amazon ranks every listing in its category by recent sales: #1 sells the most. When a unit sells, the rank jumps up (the number gets smaller), then drifts back down until the next sale. That is why the rank line in the sales rank box looks like a saw.
Drops
Each sharp jump in rank is called a drop, and a drop is usually at least one sale. Counting drops over 30 days gives a floor for sales a month: a listing with 60 drops in 30 days sold at least 60 units, and often more, since a busy listing sells several units between two readings. A listing with no drops in 30 days has barely sold.
Bought in past month
On many listings Amazon itself shows a figure such as "400+ bought in past month". It comes in steps: 50, 100, 200 and so on up to 900, then 1,000, 2,000 and up. "400+" means somewhere from 400 to just under 500, so the middle of the step, about 450, is a fairer read than the bottom. Where there is no such figure, use the drops.
Your sales, not the listing's
The listing's sales are shared by every seller who can win the Buy Box, and by Amazon when it sells too. Your sales a month are the listing's times your share. See The Buy Box and your share.
Check the trend too: if sales fell over the last 30 days, size on the lower figure. If they're low only because the price sits above its usual range, they tend to return when the price does.
The PO Assistant's suggested quantity starts from the listing's monthly sales, split evenly between you and the sellers on the listing. When there's no sales figure it says "no sales data" rather than guess, and a listing with little history gets a note such as "Only 2 months of history". See what a read gives you.
Slow listings
Slow listings tie up cash and pay storage longer. Many buyers keep a floor, counted on the listing's sales (for "400+ bought in past month", 400). The Results rules can do it for you: Rank drops in 30 days hides listings with fewer drops than your number. See What the rules test. Leave it loose if you'd rather let your profit and ROI minimums decide.
A worked example
A listing shows "300+ bought in past month" and 290 drops in 30 days. Read the middle of the step: about 350 a month. There are 4 sellers who can win the Buy Box and Amazon isn't selling, so your share is a fifth: about 70 a month for you, a bit over 2 a day. For 45 days that's 105 units, about 9 cases of 12.
If the same listing showed no bought in past month figure and 40 drops, count about 40 a month for the listing and 8 for you. That's 12 units for 45 days: a single case of 12, the least worth trying on a product that sells at all.