1 knife is 2.

Block,

1+ 1

=2;

The second knife is 4.

Block,

2+2

=

4;

The third knife is 7.

Block,

4+3

=7,

Because if the limit is cut vertically on a plane, it is impossible to cut four pieces, and at most it can only be cut like this.

The fourth knife is 13.

7+6

=

13,

. . .

Of course, if you can cut freely in space, it will be very simple.

2

,4

,8,

16, .....