Number Sequence Problem: Finding the Successor to 999

To solve this problem, let's outline the steps:

• Step 1: Recall that the successor of a number is the number plus one.
• Step 2: We are given the number 999.
• Step 3: Apply the successor formula: $n + 1$.

Now, let's apply these steps:
Step 1: We have the number 999.
Step 2: According to the successor rule, we add 1 to 999.
Step 3: Perform the calculation: $999 + 1 = 1000$.

Therefore, the number that succeeds 999 is $1000$.