When I configure ILM it will create an index with 000001, what happens when you roll over for 999999? Will it just start over again, and no worries. Only reason it's a large number is to make sure it's not the same twice?
There's no arbitrary limit, after
index-999999 the next one will be
index-1000000 and so on, internally it's an integer, so it can go up to 2147483647 indices. (I suppose we could change that to a long so it can go up to 9223372036854775807 indices, but even as an int overflowing would take a very long time.)