tenkasu ga ooi hou ga ii to ha kagiranai.

Here, ほう means something like the side / the option / the case.

So:

• 多いほう = the side that has more, or more naturally, the option with more

In English, we usually would not say the side in this kind of sentence, but Japanese often uses ほう when comparing possibilities or talking about which of two situations is preferable.

So:

• 天かすが多いほうがいい
  = the option with more tenkasu is better

Xほうがいい is a very common pattern meaning:

• it is better to choose X
• X is preferable

So here:

• 多い = many / a lot
• 多いほう = the one with more
• 多いほうがいい = the one with more is better / it is better if there is more

In this sentence, the full chunk is:

• 天かすが多いほうがいい
  = having more tenkasu is better

This is treated as one whole idea, and then とは限らない is attached to it.

They are doing different jobs.

First が: 天かすが

This marks what is many.

• 天かすが多い = there is a lot of tenkasu / the tenkasu is plentiful

Here, 多い often takes が for the thing that is abundant.

Second が: ほうがいい

This is part of the pattern Xほうがいい.

• Aほうがいい = A is better

So the sentence is basically:

• 天かすが多い = there is more tenkasu
• 天かすが多いほうがいい = the option where there is more tenkasu is better

So the two が are not redundant; they belong to different parts of the sentence.

～とは限らない means:

• it is not necessarily the case that ~
• ~ is not always true
• you cannot say that ~ in every case

So:

• 天かすが多いほうがいいとは限らない
  = It is not necessarily true that more tenkasu is better.

Important nuance: this does not mean the speaker is saying the opposite is always true.

It does not mean:

• having less tenkasu is better

It only means:

• you cannot automatically assume that more is better

The と is quoting or marking the preceding clause as a proposition: “that …”

So:

• 天かすが多いほうがいい = more tenkasu is better
• 天かすが多いほうがいいと = that more tenkasu is better

Then:

• とは限らない = is not necessarily so

You can think of it like:

• [statement] + とは限らない
• It is not necessarily true that [statement]

The は adds contrast/emphasis. It gives the sense of:

• as for saying that ..., that is not necessarily true

The は adds a contrastive nuance.

Compare the feeling:

• ～と限らない would sound less standard here
• ～とは限らない is the normal fixed expression

The は helps highlight the whole previous statement as something being questioned or limited:

• As for the idea that X, that is not necessarily true

So in practice, ～とは限らない is best learned as a set phrase.

Because this is a plain-style sentence.

The core statement is:

• 天かすが多いほうがいい = more tenkasu is better

And then:

• とは限らない = is not necessarily true

Japanese often uses plain forms inside larger grammar patterns, even in sentences that might later be made polite.

A more polite version could be:

• 天かすが多いほうがいいとは限りません。

But the version you were given is simply plain style.

Not with exactly the same nuance.

For example:

• 天かすが多いのがいい

This can mean something like:

• It is good that there is a lot of tenkasu
• The one with lots of tenkasu is good

But ほう is much more natural when talking about which option/case is preferable, especially when there is an implied comparison.

So:

• 多いほうがいい = the more-abundant option is better
• 多いのがいい = possible in some contexts, but less like a comparison between alternatives

In this sentence, ほう is the more natural choice.

Yes. ほう often implies a comparison, even if the other side is not stated.

So:

• 多いほうがいい suggests compared with having less, having more is better

The sentence then says that this comparison-based claim is not necessarily true.

In other words, the speaker is pushing back against a simple assumption like:

• More tenkasu = automatically better

In this sentence, 天かす is not marked as the topic; it is marked with が.

• 天かすが多い = there is a lot of tenkasu

So 天かす is the thing being described as abundant.

If it were the topic, you might see 天かすは, but that would make the sentence feel different.

Using が here fits naturally with 多い, because 多い commonly takes が for the thing that is numerous or plentiful.

No. That would be too strong.

～とは限らない only means:

• not necessarily
• not always
• you can't generalize that way

So the sentence means:

• sometimes more tenkasu may be better
• sometimes it may not be
• it depends

It is a cautious statement, not a direct rejection.

The reading is:

• かぎらない

The dictionary form is:

• 限る（かぎる） = to limit
• 限らない = is not limited to / is not necessarily the case

In the grammar pattern ～とは限らない, it has become an idiomatic expression meaning:

• it is not necessarily true that ~

So although the verb literally relates to limiting, you should learn the whole pattern as a unit.

You could break it down that way to understand the grammar, but it is not good English.

A rough literal breakdown is:

• 天かすが多いほうがいい = the side with more tenkasu is good
• と = that
• は = contrast/topic emphasis
• 限らない = is not necessarily the case

Natural English would be:

• More tenkasu is not necessarily better.
• It’s not necessarily better to have more tenkasu.

Those sound much more natural than a word-for-word translation.

Yes, definitely.

～とは限らない is very common in Japanese, especially in explanations, essays, discussions, and careful speech.

Useful examples:

• 高いものがいいとは限らない。
  Expensive things are not necessarily good.

• 日本人ならみんな寿司が好きとは限らない。
  Not all Japanese people necessarily like sushi.

• 努力すれば成功するとは限らない。
  If you make an effort, it does not necessarily mean you will succeed.

So this sentence is a very good model for learning a broadly useful pattern.