As Tyler James Young points out, the sentence is incomplete: there is a noun or NP missing at the end, something like:
... not important for the company's financial strength, or status, or balance sheet.
Otherwise, yes: [How many cars the company has] is the subject of the sentence. It is, however, a singular subject, a quantity, not a plural one, so the sentence should read
How many cars the company has is not important for the company's financial whatever-it-is.
Jolenealaska asks why it is singular, which is a very good question. How many cars the company has does not stand for the expression X cars, which would be plural, it stands for just the number, one number, which is singular. Imagine a conversation between the owner of the company and the accountant:
ACCT: Now here, opposite 'fleet', you have entered '12'.
OWN: Right, we have twelve cars! Up from ten last year!
ACCT: That is not important. How many you have is not important. Bob, you imbecile, I tell you this every year: the Internal Revenue Service doesn't care how many cars you have. They want to know the value of your fleet.