20,000

20,000 (twenty thousand) is the natural number that comes after 19,999 and before 20,001.

20,000 is a round number, and is also in the title of Jules Verne's 1870 novel Twenty Thousand Leagues Under the Seas.

20001 to 20999

 * 20002 = number of surface-points of a tetrahedron with edge-length 100
 * 20067 = The smallest number with no entry in the Online Encyclopedia of Integer Sequences (OEIS)
 * 20100 = sum of the first 200 natural numbers (hence a triangular number)
 * 20160 = highly composite number; the smallest order belonging to two non-isomorphic simple groups: the alternating group A8 and the Chevalley group A2(4)
 * 20161 = the largest integer that cannot be expressed as a sum of two abundant numbers
 * 20230 = pentagonal pyramidal number
 * 20412 = Leyland number: 93 + 39
 * 20540 = square pyramidal number
 * 20569 = tetranacci number
 * 20593 = unique prime in base 12
 * 20597 = k such that the sum of the squares of the first k primes is divisible by k.
 * 20736 = 1442 = 124, 1000012, palindromic in base 15 (622615)
 * 20793 = little Schroeder number
 * 20871 = The number of weeks in exactly 400 years in the Gregorian calendar
 * 20903 = first prime of form 120k + 23 that is not a full reptend prime

21000 to 21999

 * 21025 = 1452, palindromic in base 12 (1020112)
 * 21147 = Bell number
 * 21181 = the least of five remaining Seventeen or Bust numbers in the Sierpiński problem
 * 21209 = number of reduced trees with 23 nodes
 * 21637 = number of partitions of 37
 * 21856 = octahedral number
 * 21943 = Friedman prime
 * 21952 = 283
 * 21978 = reverses when multiplied by 4: 4 × 21978 = 87912

22000 to 22999

 * 22050 = pentagonal pyramidal number
 * 22140 = square pyramidal number
 * 22222 = repdigit, Kaprekar number: 222222 = 493817284, 4938 + 17284 = 22222
 * 22447 = cuban prime
 * 22527 = Woodall number: 11 × 211 − 1
 * 22621 = repunit prime in base 12
 * 22699 = one of five remaining Seventeen or Bust numbers in the Sierpiński problem

23000 to 23999

 * 23000 = number of primes $$\leq 2^{18}$$.
 * 23401 = Leyland number: 65 + 56
 * 23409 = 1532, sum of the cubes of the first 17 positive integers
 * 23497 = cuban prime
 * 23821 = square pyramidal number
 * 23833 = Padovan prime
 * 23969 = octahedral number
 * 23976 = pentagonal pyramidal number

24000 to 24999

 * 24000 = number of primitive polynomials of degree 20 over GF(2)
 * 24211 = Zeisel number
 * 24336 = 1562, palindromic in base 5: 12343215
 * 24389 = 293
 * 24571 = cuban prime
 * 24631 = Wedderburn–Etherington prime
 * 24649 = 1572, palindromic in base 12: 1232112
 * 24737 = one of five remaining Seventeen or Bust numbers in the Sierpinski problem
 * 24742 = number of signed trees with 10 nodes

25000 to 25999

 * 25011 = the smallest composite number, ending in 1, 3, 7, or 9, that in base 10 remains composite after any insertion of a digit
 * 25085 = Zeisel number
 * 25117 = cuban prime
 * 25200 = 224th triangular number, highly composite number, smallest number with exactly 90 factors
 * 25205 = largest number whose factorial is less than 10100000
 * 25482 = number of 21-bead necklaces (turning over is allowed) where complements are equivalent
 * 25585 = square pyramidal number
 * 25724 = Fine number
 * 25920 = smallest number with exactly 70 factors

26000 to 26999

 * 26015 = number of partitions of 38
 * 26214 = octahedral number
 * 26227 = cuban prime
 * 26272 = number of 20-bead binary necklaces with beads of 2 colors where the colors may be swapped but turning over is not allowed
 * 26861 = smallest number for which there are more primes of the form 4k + 1 than of the form 4k + 3 up to the number, against Chebyshev's bias
 * 26896 = 1642, palindromic in base 9: 408049

27000 to 27999

 * 27000 = 303
 * 27405 = heptagonal number, hexadecagonal number, 48-gonal number, 80-gonal number, smallest integer that is polygonal in exactly 10 ways.
 * 27434 = square pyramidal number
 * 27559 = Zeisel number
 * 27594 = number of primitive polynomials of degree 19 over GF(2)
 * 27648 = 11 × 22 × 33 × 44
 * 27653 = Friedman prime
 * 27720 = highly composite number; smallest number divisible by the numbers from 1 to 12 (there is no smaller number divisible by the numbers from 1 to 11 since any number divisible by 3 and 4 must be divisible by 12)
 * 27846 = harmonic divisor number
 * 27889 = 1672

28000 to 28999

 * 28158 = pentagonal pyramidal number
 * 28374 = smallest integer to start a run of six consecutive integers with the same number of divisors
 * 28393 = unique prime in base 13
 * 28547 = Friedman prime
 * 28559 = nice Friedman prime
 * 28561 = 1692 = 134 = 1192 + 1202, number that is simultaneously a square number and a centered square number, palindromic in base 12: 1464112
 * 28595 = octahedral number
 * 28657 = Fibonacci prime, Markov prime
 * 28900 = 1702, palindromic in base 13: 1020113

29000 to 29999

 * 29241 = 1712, sum of the cubes of the first 18 positive integers
 * 29341 = Carmichael number
 * 29370 = square pyramidal number
 * 29527 = Friedman prime
 * 29531 = Friedman prime
 * 29601 = number of planar partitions of 18
 * 29791 = 313

Primes
There are 983 prime numbers between 20000 and 30000.