कितनी बार? दिन में एक बार; दिन में दो बार।

Because बार is treated as a feminine noun in Hindi when it means time/occurrence.
So the question word कितना changes to agree with it:

• masculine singular: कितना
• feminine singular: कितनी
• plural/common counting contexts: often still कितने in many patterns

Here, बार is feminine singular, so कितनी बार? is the natural form for How many times? / How often?

In this sentence, बार means time in the sense of an occurrence or instance, not clock time.

So:

• एक बार = once / one time
• दो बार = twice / two times

This is a very common word in Hindi for frequency.

Literally, दिन में means in the day.

• दिन = day
• में = in

But in this kind of sentence, दिन में is best understood as per day / in a day / during the day.

So:

• दिन में एक बार = once a day
• दिन में दो बार = twice a day

में is the postposition meaning in. Hindi uses postpositions where English often uses prepositions.

So instead of something like in day, Hindi says:

• दिन में = in the day / during the day

In frequency expressions, this becomes the natural way to say a day / per day in context:

• दिन में एक बार = once a day

Yes, it is very natural and common.

A very standard pattern in Hindi is:

• [time period] + में + [number] + बार

Examples:

• दिन में एक बार = once a day
• दिन में दो बार = twice a day
• हफ़्ते में एक बार = once a week
• महीने में दो बार = twice a month

So this is a useful pattern to remember.

Hindi usually expresses this idea with number + बार.

So English:

• once
• twice

Hindi:

• एक बार
• दो बार

Hindi does not usually need separate special words equivalent to English once and twice. The number + बार pattern is normal and very productive.

The basic order is:

• दिन में एक बार
• literally: day in one time

More naturally analyzed as:

• during the day
  • one time

Hindi often puts the time-setting expression first, and then the frequency.

So the structure is:

[time period] + में + [number] + बार

A simple pronunciation guide:

• कितनी बार? = kit-nee baar?
• दिन में एक बार = din मेँ ek baar
• दिन में दो बार = din मेँ do baar

A bit more precisely:

• कि = short ki
• तनी = tnee with a light t
• बार = long baa-r
• दिन = din
• में = mein with a nasal sound
• एक = ek
• दो = do

The ाँ/ें-type nasalization matters, though many learners are understood even before they master it.

They are simply the numbers:

• एक = one
• दो = two

Combined with बार, they give frequency:

• एक बार = once
• दो बार = twice

This is a very common way to count repeated actions in Hindi.

Yes. In real usage, कितनी बार? can ask either:

• How many times?
• How often?

The exact meaning depends on context.

For example:

• If someone asks about medicine: कितनी बार?
  it usually means How often?
• If someone asks about repeated events in the past, it may mean How many times?

So the same Hindi phrase can cover both ideas.

Yes, absolutely. This is one of the most useful patterns for frequency in Hindi.

Examples:

• दिन में एक बार = once a day
• दिन में तीन बार = three times a day
• हफ़्ते में दो बार = twice a week
• महीने में एक बार = once a month
• साल में एक बार = once a year

So if you remember [time period] + में + [number] + बार, you can build many similar sentences.