The fact that in Revelation certain numbers (e.g. three, seven, twelve, etc.) appear frequently, and others (e.g. eight, nine or eleven) not at all, suggests to us that these numbers are symbolic. (It would be unusual, even in the providence of God, if everything that needed to be numbered occurred only in such limited range!)

Numbers in Revelation are qualitative rather than quantitative. That is to say, they tell us about the nature of things rather than their amount. In English we often use a ‘dozen’ in this way, as in “I’ve told you dozens of times”. This doesn’t mean “The number of times I’ve told you divides exactly into twelve” but “I’ve told you enough”.

The commonest numbers and their likely symbolism in Revelation are as follows:

3 – Hebrew uses repetition for emphasis – ‘Holy, Holy, Holy” = “the most holy”
3 1/2 – half of 7 (see below); also expressed as “time, times and half a time” (1 + 2 + 1/2 = 3 1/2)
4 – the whole of creation (e.g. the “four corners of the earth” [In modern times we say “four points of the compass” – North, South, East, West – to mean “everywhere”])
6 – 1 short of 7 – short of completeness/perfection
7 – the complete or perfect number, or the essence of things (e.g. “I have sailed the seven seas”)
10 – a number of magnitude (“many” or “many times” – in English we tend to use “dozens” or “hundreds”)
12 – the number of God’s people – Israel (tribes) or the church (apostles)
24 – 12 plus 12 – the combined people of God (including both Old and New Covenants)
42 – the number of months in 3 1/2 years
144 – 12 times 12 – the total number of the whole people of God
666 – work it out! (7 minus 1, 3 times for repetition)
1,000 – 10 cubed (10 multiplied by itself 3 times) – the ‘most superlative’ number (compare the numbers 10 and 3)
1,260 – the number of days in 3 1/2 years of  twelve 30 day months
144,000 – Can you work this out? What does it represent? (check out this article: http://thegospelcoalition.org/blogs/kevindeyoung/2012/01/17/who-are-the-144000-in-revelation/)

(From Revelation Unwrapped written by John Richardson, page 16)