Learn Spanish (Spain): Para la receta necesito quinientos gramos de arroz. - For the recipe I need five hundred grams of rice.

We use para because it indicates the purpose or destination of the rice. You are getting the rice specifically "for" the recipe. A helpful trick is to see if you can swap it with "in order to make" in English.

Can I change the word order and start the sentence with "necesito"?

Yes, sentence order in Spanish is quite flexible. Starting with Para la receta just places a little extra emphasis on what the ingredients are for. Saying Necesito quinientos gramos de arroz para la receta is completely correct and very natural.

Why is 500 spelled "quinientos" instead of "cincocientos"?

The number 500 is an exception. While most hundreds are formed by combining the number with cientos (like doscientos or trescientos), 500 changes its root completely to quinientos. It is an irregular spelling you just have to memorize.

Does the word "quinientos" change depending on what you are counting?

Yes! All hundreds from 200 to 999 must match the gender of the noun they count. Since gramos is a masculine plural word, we use quinientos. If you were counting a feminine word, like apples (manzanas), it would change to quinientas manzanas.