Počet najetých kilometrů je vysoký.

Počet means the number/amount/count of something.

So počet kilometrů = the number of kilometers.

By contrast, číslo usually means a numeral, number as a symbol, digit, or figure. For example:

• telefonní číslo = phone number
• číslo domu = house number

In this sentence, Czech uses počet, because it is talking about how many kilometers have been driven, not about a number as a label or symbol.

After počet, Czech normally uses the genitive:

• počet studentů = the number of students
• počet aut = the number of cars
• počet kilometrů = the number of kilometers

So kilometry would not work here, because after počet, the noun must be in the genitive.

For kilometr, the genitive plural is kilometrů.

Najetých comes from the verb najet / najíždět, which in this context means to cover by driving, to rack up, or to accumulate by traveling in a vehicle.

So:

• najet kilometry = to drive/cover kilometers
• najeté kilometry = kilometers that have been driven/covered

In the sentence:

• počet najetých kilometrů

literally means:

• the number of driven/covered kilometers

In more natural English, that is often just mileage.

Because it describes kilometrů, and adjectives must agree with the noun they modify.

Here the noun is:

• kilometrů = genitive plural

So the adjective must also be:

• najetých = genitive plural

This is standard adjective agreement in Czech.

Because the subject of the sentence is počet, and počet is a masculine inanimate singular noun.

The structure is:

• Počet ... je vysoký.
• The number ... is high.

Since vysoký describes počet, it must agree with počet, not with kilometrů.

So:

• počet = masculine singular
• therefore vysoký = masculine singular

That is why it is vysoký, even though kilometrů is plural.

Yes. In Czech, vysoký is commonly used not only for physical height, but also for things like:

• high number
• high level
• high price
• high salary

So počet ... je vysoký is completely natural and means:

• the number is high
• the mileage is high

Other examples:

• Cena je vysoká. = The price is high.
• Mzda je vysoká. = The salary is high.
• Počet případů je vysoký. = The number of cases is high.

Yes, grammatically you can. But the meaning becomes broader or less specific.

• Počet kilometrů je vysoký. = The number of kilometers is high.
• Počet najetých kilometrů je vysoký. = The number of kilometers driven is high.

Adding najetých makes it clear that these are kilometers already covered by a vehicle, so it sounds especially natural when talking about:

• a car
• a motorcycle
• a truck
• mileage in general

Without najetých, the phrase is more neutral and could depend more on context.

It is correct and natural, especially in formal or neutral language. But in everyday Czech, people very often use other expressions too, for example:

• Má najeto hodně kilometrů. = It has a lot of kilometers on it.
• To auto má hodně najeto. = That car has done a lot of miles/kilometers.
• Auto má vysoký počet najetých kilometrů. = The car has high mileage.

So your sentence is good, but it can sound a bit more formal or written than some everyday alternatives.

The neutral word order is:

• Počet najetých kilometrů je vysoký.

This is a normal subject + verb + complement structure:

• Počet najetých kilometrů = subject
• je = is
• vysoký = high

Czech word order is flexible, so other orders are possible for emphasis, for example:

• Vysoký je počet najetých kilometrů.

But that sounds more marked or stylistically special. For most learners, the original order is the safest and most natural choice.

In a normal full sentence, yes.

• Počet najetých kilometrů je vysoký.

Czech usually needs the present tense of být (to be) in this type of sentence.

In headlines, notes, or very informal styles, Czech sometimes omits it, but for standard grammar you should keep je.

By itself, the sentence is complete. It simply means that the mileage is high.

But in real use, Czech often makes the owner or object explicit:

• U toho auta je počet najetých kilometrů vysoký.
• Počet najetých kilometrů u tohoto vozu je vysoký.
• Tento vůz má vysoký počet najetých kilometrů.

So your sentence is fine on its own, but in conversation it often appears with context such as the car, the vehicle, etc.

A rough pronunciation guide:

• najetých ≈ na-ye-teehkh
• kilometrů ≈ ki-lo-me-troo

A few useful points:

• j in Czech sounds like English y in yes
• ch is the throaty sound in German Bach or Scottish loch
• ů is pronounced like a long ú

So:

• najetých has na + je + tých
• kilometrů ends with a long -trů

If you want to sound natural, keep the stress on the first syllable of each word:

• PO-čet
• NA-je-tých
• KI-lo-me-trů
• je
• vy-SO-ký

(Stress in Czech is always on the first syllable of the word.)

It is in the nominative singular, because it is the subject of the sentence.

So the sentence structure is:

• Počet = nominative subject
• najetých kilometrů = dependent genitive phrase
• je vysoký = predicate

This is a very common Czech pattern:

• Počet studentů je malý.
• Počet obyvatel roste.
• Počet najetých kilometrů je vysoký.

So even though there is a plural noun later in the phrase, the whole sentence is built around singular počet.