There donot exist a formula for counting chainisomers. However for quick remembring u can use the following concept for first 9 alkanes

For each consequetive memeber , the no of chain isomers is of  2n-1 where n is the no of isomers of preceding member

Carbon atoms - Isomers

4 - 2 (Buane has 2 isomers)

5 - 3 (2*2-1)

6 - 5 (2*3-1)

7 - 9 (2*5-1)

8 - 18 (exception 2*9)

9 - 35 (2*18-1)