Make your product buy-able in only increments

If you want your products only buy-able in quantities of 5, 10, 15 (for example), then Quantity Increment will be able to help you with that.

Set up the rule for your store

If there are several rules applied to the same products or customers, and they have the same priorities, the latest created rule is applied.

Choose who can see the altered pricing on your products

You can choose Specific customers by entering their emails
You can choose Customer tags to choose a group of eligible customers (e.g. "b2b" tag)

You can also choose who are excluded from Apply to Customers, in Exclude Customers:

Choose which products to apply the rule to.

In the 1st field (Quantity Increments), you'd enter the number to set a quantity increment.

For example, if you set this number to 5, every product is buy-able only in 5, 10, 15, etc.

In Quantity Increment Calculation, there are 3 options:

Variant Quantity: each product variant's quantity must reach the set increment to be buy-able
Product Quantity: each product's quantity must reach the set increment to be buy-able
Order Quantity: the order's quantity must reach the set increment to be buy-able

You can set up the maximum and/or minimum quantity each order has to reach.

The number(s) you put here is related to the option you choose in Quantity Increment Calculation

For example:

Quantity Limit is set to Minimum of 10
If you choose Quantity Increment Calculation to be Variant Quantity, each variant's minimum quantity is 10
If you choose Quantity Increment Calculation to be Product Quantity, each product's minimum quantity is 10

The same applies to Order Quantity.

After setting up everything, hit the Save button.

For demonstration, here's my setup:

Every time I click on the - or + button on the quantity selector, the quantity automatically jumps by 5.
If I try to input 22, for example, a warning message will be displayed under the Quantity box: