Broj okvira je ovdje.

Because after broj (number), Croatian normally uses the genitive form of the noun:

• broj telefona = phone number
• broj stranice = page number
• broj računa = account number
• broj okvira = frame number

So okvir changes to okvira.

If the intended meaning is frame number (an identifying number for one frame), then okvira is best understood as genitive singular.
A useful extra detail: okvira can also be genitive plural, so in some contexts the phrase could also mean number of frames. Context tells you which meaning is meant.

Broj is in the nominative singular because it is the subject of the sentence.

The structure is:

• Broj okvira = the subject
• je = is
• ovdje = here

So grammatically, the sentence is built around broj.

Je is the 3rd person singular present of biti (to be), and it matches the singular subject broj.

So:

• broj = singular
• therefore je = singular is

You usually need je in a normal full sentence like this. Leaving it out would sound incomplete unless you were writing a label, note, or very informal shorthand.

Croatian does not have articles like English a and the.

So Broj okvira je ovdje can mean something like:

• The frame number is here
• or, depending on context, A frame number is here

In real use, context tells you whether something is definite or indefinite.

No. Croatian word order is fairly flexible.

Broj okvira je ovdje is a normal, neutral order. But you could also say:

• Ovdje je broj okvira. = puts more focus on here
• Je li broj okvira ovdje? = Is the frame number here?

One important thing, though: je is a clitic, so it usually appears in or near the second position of the sentence or clause.

Yes, often you can.

• ovdje = here
• tu = often here or there, depending on context

In many everyday situations, Broj okvira je tu would also sound natural.

Very roughly:

• ovdje is a bit more explicitly here, in this place
• tu is very common in speech and can be slightly looser in meaning

A rough English-friendly approximation is OV-dyeh.

A few notes:

• o is like o in for but shorter and cleaner
• v is a normal v
• dj is a soft sound, roughly like dy in during for many English speakers
• e is like e in bet, but cleaner and shorter

So: OV-dyeh is a useful approximation, even though it is not exact.

Yes. This is a very common Croatian pattern:

• broj + genitive noun

It is used for many kinds of identifying numbers:

• broj telefona = phone number
• broj putovnice = passport number
• broj računa = account number
• broj okvira = frame number

So this sentence is grammatically very typical.

Yes. That is one of the tricky things here.

The phrase broj okvira can mean either:

• frame number / number of the frame
• number of frames

The form alone does not always tell you which one is meant, because okvira can match more than one genitive meaning. Usually the surrounding context makes it clear immediately.

So if the meaning has already been shown to the learner, that is important here.

Because after broj in this structure, Croatian does not use the plain nominative plural.

So you say:

• broj okvira
  not
• broj okviri

If you wanted frames itself to be the subject, then you would use the plural nominative:

• Okviri su ovdje. = The frames are here.

But in your sentence, the main noun is broj, not okviri.