Quiero conservar el número en el móvil de prepago.

After querer (to want), Spanish normally uses an infinitive for the next verb.

• quiero conservar = I want to keep / retain
• not quiero conservo

This is the same pattern as:

• Quiero comer = I want to eat
• Quiero llamar = I want to call
• Quiero cambiar de tarifa = I want to change tariff/plan

So quiero is the conjugated verb, and conservar stays in the infinitive.

Because Spanish often drops subject pronouns when they are not needed.

• Quiero conservar... already clearly means I want to keep...
• The ending -o in quiero shows the subject is I

You can say yo quiero if you want extra emphasis or contrast:

• Yo quiero conservar el número, pero ella no.
  = I want to keep the number, but she doesn’t.

In a normal neutral sentence, quiero on its own is more natural.

Conservar here means to keep, to retain, or to preserve something you already have.

In the context of a phone number, conservar is a good choice for keeping the same number.

Comparisons:

• conservar = retain / keep
• mantener = maintain / keep up
• guardar = keep/store/put away

So:

• conservar el número sounds like keeping the number
• guardar el número would more likely mean saving the number in your contacts
• mantener el número can work, but conservar is very natural for retaining it

Spanish often uses the definite article (el, la, los, las) where English might use a possessive.

So el número can still mean my number, if the context makes that obvious.

This happens a lot in Spanish:

• Me duele la cabeza = My head hurts
  literally: The head hurts me
• Lávate las manos = Wash your hands
  literally: Wash yourself the hands

Here, el número is fine if everyone already knows whose number is being discussed.
If you want to make it more explicit, you can also say:

• Quiero conservar mi número...

That is also perfectly correct.

Yes. In Spain, móvil is the usual word for mobile phone / cell phone.

• Spain: móvil
• Much of Latin America: celular or móvil, depending on the country

So a learner of Spanish from Spain should definitely know móvil.

Examples:

• No tengo batería en el móvil.
• Te llamo al móvil.

If you said celular in Spain, people would understand you, but móvil is more natural there.

De prepago means prepaid.

It literally means something like of prepayment / prepaid type. Spanish often uses de + noun to describe things in this way.

So:

• móvil de prepago = prepaid mobile phone / prepaid mobile service
• tarjeta de prepago = prepaid SIM card / prepaid card
• línea de prepago = prepaid line

In real-life phone-company language, prepago is very common in Spain.

Here en means something like on / in / with, depending on how you interpret the context.

The idea is that the number is being kept on that prepaid mobile / prepaid line.

Spanish often uses en in places where English might choose different prepositions. However, this part can feel a little context-dependent, and in some situations a Spanish speaker might choose a slightly different wording, such as:

• Quiero conservar el número en la línea de prepago.
• Quiero conservar el número en la tarjeta de prepago.

If you are specifically talking about the phone itself, en el móvil de prepago is understandable, but if you mean the phone service or SIM, línea or tarjeta may sound more precise.

It is understandable, but depending on the exact situation, a native speaker might choose a more specific phrase.

Common possibilities in Spain include:

• móvil de prepago = prepaid mobile phone
• tarjeta de prepago = prepaid SIM/prepaid card
• línea de prepago = prepaid line

So if the real meaning is about keeping your phone number while staying on a prepaid plan, some very natural alternatives could be:

• Quiero conservar el número en una línea de prepago.
• Quiero conservar mi número de prepago.
• Quiero mantener mi número en prepago.

Your original sentence is understandable, but línea or tarjeta may sound more exact than móvil, depending on what you really mean.

Because in normal Spanish word order, the verb usually comes before its direct object.

• conservar el número = keep the number

This is the standard order:

• Quiero cambiar la tarifa
• Necesito recargar el saldo
• Voy a conservar el número

Putting el número before conservar would usually sound wrong in a basic sentence like this.

Yes. Those are possible alternatives, though they are not identical in tone.

• conservar el número = retain the number
• mantener el número = keep/maintain the number
• seguir con el mismo número = continue with the same number

All three can work, but:

• conservar sounds slightly more formal or precise
• mantener is also natural
• seguir con el mismo número sounds very everyday and conversational

Examples:

• Quiero conservar el número.
• Quiero mantener mi número.
• Quiero seguir con el mismo número.

It is fairly neutral.

Nothing in it is especially formal or especially casual:

• quiero = neutral everyday language
• conservar = slightly more careful/formal than seguir con
• móvil = normal Spanish from Spain
• prepago = standard phone-service vocabulary

So the sentence would fit well in situations like:

• talking to customer service
• explaining your situation in a shop
• writing a message about your phone plan

If you wanted it to sound more casual, you might say:

• Quiero seguir con el mismo número en prepago.

If you wanted it slightly more customer-service-like, you might say:

• Quiero conservar mi número en una línea de prepago.