The answer is $\boxed{\text{12}}$.
The sequence is a list of numbers that alternate between increasing by 1 and increasing by 3. The first number is 2, and the second number is 1. This is a decrease of 1, so the next number in the sequence is 2 + 1 = 3. The next number is 4, which is an increase of 1. The next number is 4, which is a repetition of the previous number. This is because the sequence alternates between increasing by 1 and increasing by 3, and the previous number was an increase of 1. The next number in the sequence is 5, which is an increase of 1. The next number is 6, which is an increase of 1. The next number is 7, which is an increase of 1. The next number is 8, which is an increase of 1. The next number is 8, which is a repetition of the previous number. This is because the sequence alternates between increasing by 1 and increasing by 3, and the previous number was an increase of 1. The next number in the sequence is 10, which is an increase of 2. The next number is 11, which is an increase of 1. The next number in the sequence is $\boxed{\text{12}}$, which is an increase of 1.
Here is a table that shows the sequence:
Number | Explanation
——- | ——–
2 | First number in the sequence
1 | Decrease of 1 from the previous number
2 | Increase of 1 from the previous number
4 | Increase of 1 from the previous number
4 | Repetition of the previous number
5 | Increase of 1 from the previous number
6 | Increase of 1 from the previous number
7 | Increase of 1 from the previous number
8 | Increase of 1 from the previous number
8 | Repetition of the previous number
10 | Increase of 2 from the previous number
11 | Increase of 1 from the previous number
12 | Increase of 1 from the previous number