اگر مهمان ها زیاد باشند، باید دو کیک بیاوریم.

اگر means if. It introduces the condition:

اگر مهمان‌ها زیاد باشند = if there are many guests / if the guests are numerous

So the whole sentence has the usual if X, then Y structure.

Because this sentence talks about a possible situation, not a confirmed fact.

After اگر, Persian often uses the present subjunctive for situations that are uncertain, hypothetical, or in the future.
So:

• باشند = they be / if they are
• هستند = they are as a straightforward statement

So اگر مهمان‌ها زیاد باشند sounds natural for if there are many guests.

باشند is a form of بودن (to be).

Here it is the third person plural subjunctive form, meaning they be or if they are.

Breakdown:

• باش- = subjunctive stem
• -ند = they

It agrees with مهمان‌ها because guests is plural.

Persian often does not need a separate structure like English there is / there are.

Instead of saying something literally like if there are many guests, Persian says something closer to:

if the guests are many

So:

• مهمان‌ها = the guests / guests
• زیاد باشند = are many

That is a very normal Persian way to express this idea.

مهمان means guest.
ها is a common plural marker, so:

• مهمان = guest
• مهمان‌ها = guests

In standard modern writing, this is usually written with a half-space:

مهمان‌ها

But many learners also see:

• مهمان ها
• مهمانها

They all mean the same thing, though مهمان‌ها is the preferred standard spelling.

Yes. Both are possible:

• مهمان‌ها
• مهمانان

Both mean guests.
In everyday speech and writing, مهمان‌ها is usually more common and natural.
مهمانان can sound a bit more formal or literary.

Because زیاد here works like many / numerous, and Persian commonly puts this kind of descriptive word after the noun.

So:

• مهمان‌ها زیاد باشند = the guests are many

This is more natural in Persian than trying to place زیاد before the noun in this sentence.

باید means must, should, or have to, depending on context.

In this sentence, it most likely means something like:

• we should bring two cakes
• we need to bring two cakes

It can sound softer or stronger depending on tone and situation. It does not always have the full force of English must.

After باید, Persian usually uses a verb form that looks like the subjunctive/present stem form.

So:

• dictionary form: آوردن = to bring
• in the sentence: بیاوریم = that we bring / for us to bring

This is the normal pattern:

• باید بروم = I should go
• باید بخوانیم = we should read
• باید بیاوریم = we should bring

-یم marks we.

So بیاوریم means we bring / we should bring in this kind of structure.

Very roughly:

• بیاور- = bring
• -یم = we

That is why the sentence means we should bring two cakes.

After numbers in Persian, the noun is usually left in its basic form, not made plural with ها.

So Persian says:

• دو کیک = two cakes
• سه کتاب = three books
• پنج مهمان = five guests

Using ها after a number would usually sound wrong here.
So دو کیک is exactly what you expect.

Yes, usually the noun stays unmarked after a number.

So even though English says two cakes, Persian normally says دو کیک, not a plural-marked form.

This is a very common pattern in Persian and something English speakers often need to get used to.

Yes. Persian can also put the main clause first.

So these are both possible:

• اگر مهمان‌ها زیاد باشند، باید دو کیک بیاوریم.
• باید دو کیک بیاوریم اگر مهمان‌ها زیاد باشند.

The version with اگر... first is often clearer and more natural in careful speech or writing.

The comma is helpful, especially in writing, because it separates the if-clause from the main clause.

• اگر مهمان‌ها زیاد باشند، باید دو کیک بیاوریم.

In informal writing, people may sometimes omit it, but using it is a good habit.

It most naturally refers to a future possibility:

If there are many guests, we should bring two cakes.

Because of اگر plus باشند, the sentence sounds like a plan for a possible situation, not a description of what is definitely true right now.

Not exactly too many.
زیاد just means many, a lot, or numerous.

If you specifically wanted too many, Persian would usually make that clearer with another word, such as خیلی زیاد or another stronger expression depending on context.

So here the meaning is simply: If there are many guests...

Persian uses آوردن (to bring) when the movement is toward the relevant place or event from the speaker’s point of view.

So دو کیک بیاوریم means bring two cakes.

If the perspective were different, Persian might use بردن (to take).
This works much like English:

• bring = toward
• take = away

In this sentence, bring is the natural choice.