Un sobre vacío cuenta como voto en blanco.

Because sobre is a masculine noun in Spanish when it means envelope, so it takes un.

• un sobre vacío = an empty envelope

Be careful: sobre can also be a preposition meaning about / on / over, but in this sentence it is clearly the noun envelope.

Here, sobre means envelope.

It is a common Spanish word with more than one use:

• un sobre = an envelope
• sobre la mesa = on the table
• un libro sobre historia = a book about history

In this sentence, because it follows un and is described by vacío, it has to be the noun envelope.

In Spanish, adjectives often come after the noun, especially when they describe a physical or objective quality.

So:

• un sobre vacío = an empty envelope

That word order is the normal one here. Putting the adjective before the noun would sound unusual or more stylistic.

Also, the adjective agrees with the noun:

• un sobre vacío
• una caja vacía
• unos sobres vacíos

The accent mark shows the stress: va-CÍ-o.

Without the accent, Spanish pronunciation rules would suggest a different stress pattern. The accent also helps show that í-o is pronounced as two syllables, not merged into one.

So vacío is pronounced roughly as ba-SEE-o.

Contar como means to count as or to be counted as.

So:

• Un sobre vacío cuenta como voto en blanco
  = An empty envelope counts as a blank vote

This is different from es (is).
Es would identify what something is by nature, while cuenta como means it is treated as or considered as in a rule, system, or calculation.

Because the subject is singular: Un sobre vacío.

So the verb must also be singular:

• Un sobre vacío cuenta... = An empty envelope counts...

If the subject were plural, you would use cuentan:

• Los sobres vacíos cuentan como voto en blanco
  = Empty envelopes count as blank votes

After como, Spanish often omits the article when talking about a role, category, or classification.

So:

• cuenta como voto en blanco = counts as a blank vote

This is very natural Spanish. You may also hear como un voto en blanco, but without the article sounds more idiomatic in this kind of formal or rule-based statement.

Voto en blanco is the standard expression in Spanish for blank vote.

The phrase en blanco literally means blank, left empty, or unmarked. It is used in many fixed expressions:

• firmar en blanco = to sign a blank document
• dejar algo en blanco = to leave something blank
• voto en blanco = blank vote

So this is something you should learn as a set phrase.

No. In Spain, they are different.

• voto en blanco: a valid vote with no candidate chosen
• voto nulo: an invalid vote

So the sentence is telling you that an empty envelope is treated as a blank vote, not a null vote.

That is an important distinction in Spanish election vocabulary.

Yes, that is grammatically possible, and native speakers would understand it.

However, cuenta como voto en blanco sounds more natural and more idiomatic in this kind of general or official statement. Omitting the article after como is very common when you mean classification.

So:

• cuenta como voto en blanco = more standard here
• cuenta como un voto en blanco = possible, but slightly less natural in this context

The present tense here expresses a general rule or general fact.

• Un sobre vacío cuenta como voto en blanco
  = An empty envelope counts as a blank vote

Spanish often uses the present this way, just as English does, when stating laws, definitions, instructions, or general truths.