1. Existing number series : See if the given series is one of the existing number series.
Eg: Prime number, Triangular number, Square number
https://www.patternguru.com/series/

2. Existing Series - changed a bit : It would be one of the default number series with some small changes in it.
Eg: Square numbers - 1 , 4 , 9 ,16 , 25 ,..... There could be a question 2 , 5 , 10 ,17 ,26 ,............

3. Simple series-Addition: Find the difference between numbers and find a pattern there
Eg : 7, 8, 12, 21, 37, _ , _ ,....................

4.Simple series - mulitplication : find the multiplication factor between numbers and find a pattern there
Eg : 1,3,15,105,945,_,_,.......

5. Simple series - mixed: It could be a combination of two patterns mentioned above.
Eg : 5 , 6 , 12 , 11 , 12 , 24,23, _ ,_ ,................

5 ,    6 ,   12 ,   11 ,   12 ,  24,   23, _ ,_
   +1    *2     -1    +1     *2    -1

6. Simple series - reverse form: Sometimes we have to take the reverse form of numbers before we try to find any pattern there.
Eg : 6, 8 ,1 , 21, 41, _,.............

7. Consider the exponential number
Eg: 0, 3 , 8 , 15 , 24 , 35 , 48 ,_ ,_ , .............
Tn=(n *n) -1
T1=(1 *1) -1 = 0
T2=(2*2) -1 =3
T3=(3 *3) -1=8

8. Consider adjacent numbers and find a pattern there
Eg : 34 , 14 , 54 , 18 , 93, 24 ,98 ,_ ,.................
14 = (3 + 4) * 2
18 = (5 + 4) * 2
24 = (9 + 3) * 2

9. Divide the numbers in to groups and find a pattern there
Eg: 3 , 2 , 4 , 9 , 6,5 , 8, 19 , 8 , 7 ,11 ,_ ,.............
3 , 2 , 4 , 9 , 6, 5 , 8, 19 , 8 , 7 ,11 ,_ ,.............

9 = 3 + 2 + 4
19 = 6 + 5 + 8

10. Divide digits in each number in to two or three parts and find a pattern there
Eg : 88, 1661, 3223 , 6446 , _ , _, ....

For each and every number in the series , the second half is the reverse form of first half.
8 ---> 8
16--->61
32--->23

11. Consider individual digits in each number and find a pattern there
Eg : 8179 , 7186 , 6157 ,5164 ,4135 , _, _

8179 , 7186 , 6157 ,5164 ,4135 , _, _

8179 ---- 7=8-1 9=8+1 - , +

7186----- 8=7+1 6=7-1 +, -

Combination of multiple series : A number series which can be split in to two or three or may be more series where we can find different pattern in each of those series.
Eg : 10,1,12,3,16,7,24,6,_,_,.........

Above series is a combination of two series

10, _, 12, _, 16, _, 24, _, _, _,.......
_  ,1,   _,   3,   _, 7,   _, 6, _ ,_,.........

All the above mentioned techniques used in single series can be used in multiple series as well.

1. Map alphabets to numbers
A,   B,  C,   D,   E,   F,  G,  H,  I,   J,    K,   L,    M,    N,   O,   P,    Q,    R,   S,    T,    U,   V,    W,    X,   Y ,   Z
1,   2,   3,   4,   5,   6,  7,  8,  9,  10, 11,  12,  13,  14,  15,  16,   17,  18   19,  20,  21,  22,  23,  24,  25,  26

Eg: Assume we can denote [Orange] by the following number code, 151811475 what will be the code for [Apple] ?

2. Encrypted text/code words - A word is denoted by another word using some pattern.
Eg : If [be ready] can be encrypted as [yv ivzwb] what would be the encrypted text of the text [on the way]?
In this example, letters are mapped with reverse order of them.

a  b  c  d  e  f  g  h  i  j  k  l  m  n  o  p  q  r  s  t  u  v  w  x  y  z
z  y  x  w v  u  t  s  r  q  p o  n  m l   k   j  i   h g  f   e  d  c   b a