Buber from Poland writes:
  I'm a little confused because I've found the word thrice in a book. A few teachers told me there is no such word or that I cannot use it and that I should use three times instead. Can I use this word and in what circumstances?
Roger Woodham replies:
We’ll take this opportunity to look at a number of complications with the expression of numbers and frequency in English.
once, twice, thrice
The norm here is to say once (rather than one time) to say three times (rather than thrice) in current usage. Thrice is definitely old-fashioned, although you may still come across it in certain contexts:
This vehicle travels at thrice the speed of sound.
They play football thrice weekly.
Better to say:
This vehicle travels at three times the speed of sound.
They play football three times a week and train every night.
When it comes to twice, this is more often used than two times, although two times is also quite common in informal usage. Compare the following:
I've visited her twice already this autumn and she's visited me once.
I've visited her two times already this autumn and she's visited me once.

Unemployment in the north of England is twice the national average.
Teachers say they would be twice as effective if they had no administrative tasks.
One time is occasionally possible as an alternative to once. Compare the following:
He had only ever seen his great-aunt once before.
He had only ever seen his great-aunt one time before.

We go out with our colleagues for a drink once a week or once a fortnight and have a staff party once a year.

You will hear the recording only once.
I'm only going to say this once.
Once, (not one time) can also mean at some time in the past:
I once ran a fish-and-chip shop in Brighton. ~ When was that? ~ Before I bought this business.

Our house in the village was once the train station. ~ When was that? ~ When the trains used to run here.
Do you know the different references to these numbers?

nought / zero / nil / o / love (0)
half a dozen (6 or approximately 6)
a dozen (12 or approximately 12)
a score (20 or approximately 20)
a billion (1,000,000,000 or a very large number)

Note how they are used:
House prices rose by nought point two per cent last month.
Visibility was almost zero at the City Airport last night because of the fog.
England won their recent match against Liechtenstein by two goals to nil.
My phone number is o two o, eight seven o seven, nine nine o three.
Roddick was leading by two sets to love and forty love in the first game of the third set when rain stopped the match.

Can you set out the arguments for and against capital punishment in half a dozen paragraphs?
I bought two dozen eggs but we've only used four. ~ Why didn't you buy half a doxen?
Scores of volunteers offered to help in the search for the missing child.

How many zeros do I write down for a billion? Is it six or nine?
Billions of dollars need to be invested to re-build this country.
I've told you billions of times to lock the door before you go to bed.
maximum / minimum; maximal / minimal
To express the idea of the largest amount possible, we would normally use maximum as both adjective and noun. Maximal as adjective or maximally as adverb are more rarely used. Compare the following:
Arsenal now head the Premiership table with maximum points from five games.
The maximum sentence for armed robbery is twenty years.
How long are the shifts for this type of work? ~ Four hours is the minimum and twelve hours is the maximum.
How long should I sit in front of the computer screen? ~ Maximally three hours.
Minimal, however, meaning very small in quantity, is much more often used as an adjective. Note the slight difference in meaning. Mimimum describes the smallest amount possible. Compare the following:
He managed to pass all his exams with minimal effort.
There may be one or two delays on this service but they are expected to be minimal.

The minimum wage in Britain is now four pounds fifty an hour.
The minimum height for a policeman used to be five foot ten.