May using formula like this

in A1 and pull formula down

the first number 1 will change to 2 after 100 rows while numbers from the right will increase by one

=INT((ROWS(A$1:A1)-1)/100)+1&TEXT(ROWS($A$1:A1),"000000")

A
1
 1000001
2
 1000002
3
 1000003
4
 1000004
5
 1000005
6
 1000006
7
 1000007
8
 1000008
9
 1000009
10
 1000010
11
 1000011
12
 1000012
13
 1000013
14
 1000014
15
 1000015
16
 1000016
17
 1000017
18
 1000018
19
 1000019
20
 1000020