mjaschen

Often it’s required to limit the cart to one item. This plugin achieves this by emptying the cart before 
a new item is added.
<h3>Requirements</h3>
<ul>
<li>PHP 5.6</li>
<li>WordPress >= 4.9.0</li>
<li>WooCommerce 3.x</li>
</ul>

WooCommerce – Allow Only One Item in Cart

This WordPress plugin ensures that only one item can be in the WooCommerce cart at any given time.

elklab

This simple plugin allows to set a minimum number of items per order. 
If the number of items in the cart is less than the set value 
an error message will be shown and the checkout will be blocked.
This plugin features Italian and French translations. 
Spanish and German translations will be added in next version.
Configuration options are available in 
WooCommerce->Settings->Products->ELK Minimum Items per Order

ELK Minimum Items per Order for WooCommerce

This simple plugin allows to set a minimum number of items per order

WooCommerce – Allow Only One Item in Cart vs ELK Minimum Items per Order for WooCommerce