L’assurance demande aussi le numéro du colis et l’adresse de l’expéditeur.

Because French uses elision before a vowel sound.
The articles le and la shorten to l’ before words that begin with a vowel or mute h.

So:

• la assurance → l’assurance
• la adresse → l’adresse

This is done for smoother pronunciation.

Because French demander does not work exactly like English to ask for.

In French, you usually say:

• demander quelque chose = to ask for something
• demander quelque chose à quelqu’un = to ask someone for something

So in this sentence, demande takes its objects directly:

• le numéro du colis
• l’adresse de l’expéditeur

English needs for, but French does not here.

Aussi means also or too, and in French it is often placed after the conjugated verb.

So:

• L’assurance demande aussi... = The insurance company also asks for...

This is a very normal position for aussi in a simple sentence.
Putting it elsewhere can change the emphasis slightly, but this placement is the most natural here.

Because de + le contracts to du in French.

So:

• de le colis → du colis

This is a standard contraction:

• de + le = du
• de + les = des

That is why you get le numéro du colis = the number of the package.

Because expéditeur begins with a vowel sound, so French uses de l’ before it.

Compare:

• de + le colis → du colis
• de + l’expéditeur stays de l’expéditeur

You only use du when the noun would normally take le and does not begin with a vowel sound. Since expéditeur becomes l’expéditeur, the phrase is de l’expéditeur.

French normally repeats the article before each noun.

So French prefers:

• le numéro du colis et l’adresse de l’expéditeur

rather than leaving the second article out.

In English, we can sometimes say the package number and sender address without repeating words in the same way, but French usually sounds more natural when each noun keeps its own article.

Because numéro and nombre are not the same.

• numéro = a number used as an identifier
  
  • a phone number
  • a house number
  • a tracking number
• nombre = number in the sense of quantity or amount

Here, the package has an identifying number, so numéro is the correct word.

They are both complements introduced by de that specify the nouns before them.

• le numéro du colis = the package’s number / the number of the package
• l’adresse de l’expéditeur = the sender’s address / the address of the sender

So:

• numéro is the main noun, and du colis tells you which number
• adresse is the main noun, and de l’expéditeur tells you whose address

This is a very common French structure: noun + de + noun.

Expéditeur is the masculine form, and it can also be used in a general sense if the sender is not specified.

The feminine form is expéditrice.

So you may see:

• l’adresse de l’expéditeur = the sender’s address
• l’adresse de l’expéditrice = the female sender’s address

In many practical contexts, especially forms and instructions, French often uses the masculine or a general label unless the person’s gender matters.

A learner might be tempted to build something like demande pour le numéro, but that is not idiomatic here.

Natural French is:

• demander le numéro
• demander l’adresse

So the sentence correctly says:

• L’assurance demande aussi le numéro du colis et l’adresse de l’expéditeur.

That is one of the important differences between English and French: French demander often takes a direct object where English uses ask for.