Actually this is 100% custom modification and we don't provide free support or step-by-step instructions for custom coding modifications. But yours is easy and here it is;
Open this CSS file:
/templates/YOUR_TEMPLATE/html/com_virtuemart/assets/css/flexibleVM2Global.css
and include this CSS rule:
.product-related-products .product-field.product-field-type-R { display:none;}
.product-related-products .product-field.product-field-type-R:nth-child(2) { display:block;}
.product-related-products .product-field.product-field-type-R:nth-child(3) { display:block;}
.product-related-products .product-field.product-field-type-R:nth-child(4) { display:block;}
the above code means; Show only 3 related items. If you want to show 6 items, then use this code:
.product-related-products .product-field.product-field-type-R { display:none;}
.product-related-products .product-field.product-field-type-R:nth-child(2) { display:block;}
.product-related-products .product-field.product-field-type-R:nth-child(3) { display:block;}
.product-related-products .product-field.product-field-type-R:nth-child(4) { display:block;}
.product-related-products .product-field.product-field-type-R:nth-child(6) { display:block;}
.product-related-products .product-field.product-field-type-R:nth-child(7) { display:block;}
.product-related-products .product-field.product-field-type-R:nth-child(8) { display:block;}
regards.
Support