In English, 'broccoli' has no plural, because it is an uncountable noun. When a word enters one European language from another, it generally gets absorbed into the grammar of that language. 'Broccoli' and 'spaghetti' may be plural in the original Italian, but in English they're treated as uncountable (or 'mass') nouns. It's the same as 'rice', any other uncountable noun. No plural! You don't say 'a broccoli' and you don't say 'broccolis'. If you were to say either of those, we'd presume were referring to different types/varieties of broccoli, but this would be a fairly unusual usage. Broccoli doesn't need a plural. Some broccoli. Some more broccoli. A little broccoli. A lot of broccoli. But what if you really want to count broccoli? Well, if you're shopping, you could ask, for example, for two 'heads of broccoli'. That is generally how broccoli is sold - in 'heads' of about 10 diameter. When preparing broccoli, you would break it up into individual 'florets', which are the flower-shaped pieces that the head is made up of. Cauliflower also has florets, by the way. That's all a little technical, though. In other cases, you might say 'pieces of broccoli'. Here's a typical dinner conversation: Have some broccoli. I don't want any broccoli. I hate broccoli! Well, just have a little. How about three pieces of broccoli? One piece of broccoli. Two pieces? If you have two pieces of broccoli, I'll let you have your dessert. OK, two pieces of broccoli.

The word is Italian and already plural. I can't say that I have used or heard the word "broccolis" here in Australia. Generally you would ask "do you have any broccoli today" or "broccoli was going cheap today, so I bought three kilos".

You just call it Broccoli, there is no plural form for it.

Broccolis or just broccoli are both correct.