"limited stock" is used when the number of products is limited. How can I describe when the products have enough stock?
Can I write "plenty stock"?
"Plenty stock" doesn't sound natural... you could say "plenty of stock", but it depends on how you use it. Saying "we have plenty of stock of [x]" wouldn't be correct; we would instead say "we have plenty of [x] in stock".
The antonym of 'limited' is technically unlimited; however, no stock is really "unlimited".
Really, we state that stock is "limited" to indicate that it could easily run out. If it is not stated that stock is limited, it is normally assumed that the stock levels are reasonable to supply demand. There is really no need to look for an opposite, just don't say that the stock is limited!