Tengo más libros de los que necesito.

Because más means more, while muchos means many / a lot of.

This sentence is making a comparison:

• Tengo muchos libros = I have a lot of books.
• Tengo más libros de los que necesito = I have more books than I need.

So más is the correct word because the idea is more than necessary, not just a lot.

In Spanish, after a comparative like más or menos, than is often expressed with de lo que or de los/las que, depending on what follows.

Here, de los que necesito means literally something like than the ones that I need.

So:

• más libros de los que necesito = more books than I need

You cannot normally say Tengo más libros que necesito for this meaning. That sounds incomplete or unnatural in standard Spanish.

Because los refers back to libros, which is:

• masculine
• plural

So the structure agrees with libros:

• libros → los que
• revistas → las que
• a general idea or whole clause → lo que

Compare:

• Tengo más libros de los que necesito. = I have more books than I need.
• Gasto más de lo que debo. = I spend more than I should.

In the second sentence, there is no plural noun being referred back to, so lo que is used instead.

Here los que means the ones that or those that.

So the sentence can be unpacked like this:

• Tengo más libros de los que necesito
• literally: I have more books than the ones that I need

In natural English, we usually do not say it that way. We simply say:

• I have more books than I need.

But understanding los que as the ones that helps explain why the article los appears.

The object is understood from the earlier noun libros.

Spanish often avoids repeating a noun when it is already clear. So:

• los que necesito already means the ones that I need

You could think of the full idea as:

• Tengo más libros de los libros que necesito

But that would be repetitive and unnatural. Spanish instead uses los que to refer back to libros.

Because Spanish usually drops subject pronouns when the verb ending already makes the subject clear.

• Tengo already means I have
• the -go ending shows it is yo

So Yo tengo is possible, but it is usually only used for emphasis, contrast, or clarity.

For example:

• Tengo más libros de los que necesito. = neutral
• Yo tengo más libros de los que necesito, pero tú no. = emphasis/contrast

Not for this sentence in standard Spanish.

To say more books than I need, Spanish normally uses:

• más libros de los que necesito

Using only que after más libros is not the standard structure here.

However, que is used directly in some comparisons, especially with numbers:

• Tengo más de veinte libros. = I have more than twenty books.
• Es más alto que yo. = He is taller than me.

So the correct pattern depends on what comes after the comparison.

Yes, and it follows the same pattern.

• Tengo más libros de los que necesito.
• Necesito menos libros de los que tengo.

Both use a comparative (más / menos) plus de los que because they compare a quantity of books with another quantity related to the same noun.

Because the sentence is talking about a present situation:

• I have more books than I need right now / in general.

If the time changed, the verb could change too:

• Tenía más libros de los que necesitaba. = I had more books than I needed.
• Tengo más libros de los que necesitaré. = I have more books than I will need.

So the tense depends on the meaning you want.

The word order is normal and natural:

• Tengo = I have
• más libros = more books
• de los que necesito = than I need

Spanish often keeps this kind of comparative phrase together after the noun:

• más libros de los que necesito
• menos dinero del que esperaba
• más tiempo del que tengo

So the structure is very useful to learn as a chunk: más/menos + noun + de lo(s)/la(s) que + verb

You choose the form based on the noun being referred to.

• masculine singular → del que
• feminine singular → de la que
• masculine plural → de los que
• feminine plural → de las que

Examples:

• Tengo más dinero del que necesito.
  (dinero = masculine singular)

• Tengo más ropa de la que necesito.
  (ropa = feminine singular)

• Tengo más libros de los que necesito.
  (libros = masculine plural)

• Tengo más camisas de las que necesito.
  (camisas = feminine plural)

This agreement is one of the main reasons the phrase looks more complicated than the English than I need.

Yes, absolutely. It is a very common Spanish pattern.

Examples:

• Comí más de lo que quería. = I ate more than I wanted.
• Tengo más trabajo del que esperaba. = I have more work than I expected.
• Compró más manzanas de las que necesitaba. = She bought more apples than she needed.
• Gastamos más dinero del que debíamos. = We spent more money than we should have.

So the key thing to learn is not just this one sentence, but the broader pattern:

más/menos + noun + de + article + que + verb

or, more generally,

más/menos + de lo que + verb
when no specific noun is being referred back to.