Product Limit For A Coin

It means that the Mint can mint as many of the coin as it can sell, but can only make 20K of the particular product (i.e., set or special package) that includes the coin.

For example, the Mint is currently selling a 2019 Limited Edition Silver Proof Set with a Product Limit of 50,000. It contains, among other coins, a 2019 Proof Silver Eagle struck at the San Francisco mint. While only 50,000 of the LE sets containing the proof ASE can be produced and sold, there is no mintage limit on the proof SF ASE so the mint can strike as many of the coin as it can sell and package it in as many other ways as it wants.

So that means I will be buying 1 of 50000 of this set?

Here's one for you. $1 Year of the Rat:
Item Number: B15055
Mintage Limit: 108,888
Product Limit: 93,888
Household Order Limit: 999

I see no other products containing the dollar bill, so what explains the 15,000 difference?

$1 fortune Note (Item Number: B1514) has the same 8888 beginning serial number

Maybe 15k left for them

First off, mintage is a coin term and not a currency term. When it comes to this type of currency only product it really should refer to how many were printed and not indicate a mintage. The combined US Mint and BEP ecommerce site is still fairly new, so they may eventually start using the correct terminology for currency products, or maybe not.

For this particular product, the description doesn't indicate what series of note will be included, but for most series, due to the many different block letters used in printing the notes, there are typically many more than 108,888 8888#### serial numbered notes available (for you coin only collectors, the block letters for $1 FRN notes are the two letters before and after the numbers in the serial number).

For example, for series 2017 $1 FRNs there have been 29 different block letters with 8888#### serial numbers printed so far and more could yet be printed (the BEP was still printing 2017 series notes as of last month, but they're currently printing 2017A series notes for some FRBs as well, so the switch over has probably occurred). That means there are 290,000 2017 series $1 FRNs with 8888#### serial numbers. Way more than the supposed 108,888 mintage limit.

This product most likely is including series 2017 notes, but if they're using other series as well then the potential number available goes up even more. It all depends what the BEP pulled from the printings for use in this product. All the website indicates is that the notes may be from either print plant and any FRB which makes for potentially a very large number of notes to draw from.

In any event, I suspect that the limits indicated on the Mint's website aren't a typo and do mean something to the Mint's marketing department, but they don't in any way indicate how many 8888#### serial numbered notes could be used for this product.

Perhaps, but I suspect it's just because they're too lazy or indifferent to use the correct terms for a BEP only product.