Multiple Products or One
When it comes to using your merch designs, you have a couple options: you can put the same design on multiple different products or you can put the design on a single product as a special offering.
One Design, Multiple Products
For most designs, I am a big proponent of offering the same design on multiple products because:
you can offer more products, and try different products, without having to create a new design for them
it gives your readers a variety of product options and price points to choose from
you can launch new merch throughout the year on more seasonal products without making a new design
I generally recommend offering designs on apparel, blankets, drinkware, tote bags, and/or notebooks and then maybe choosing a fun or unique product--socks, beanies/hats, puzzles, outdoor wear, tech accessories--that ties into the design or the book it is referencing.
One Design, One Product
There is a place for offering a unique design on one (or two) products:
if you want to try more unique products that need a more specialized design
if you want to celebrate a book anniversary, a special occasion, or a holiday with a particular design and product.
One popular limited time option is to have a special Christmas ornament design that is custom for that year and only available during the holidays. This can build a fun tradition to collect each year's design.
So depending on the situation and the design, you can choose to put a design on lots of products or just a few to have a special limited offering. Check out my posts about types of merch designs and recommended merch products to learn more.