Theorem
An infinite board knight cover with 9 squares per knight
is not possible.

Proof
This can only happen if knights cover disjoint sets of 9 squares.
That is, there can't be any squares that are over-covered.

To show that this is not possible, here is an attempt to cover a
board without an over-covered square.

Place the first knight on the W-square and consider covering the
o-squares of Figure 1.  We can't occupy two of the o-squares because
the two knights will have attacked squares in common.
So at least one o-square must be covered by attack.
The x-squares are the only locations that can attack an
o-square while avoiding squares attacked by the W knight.

- - - - -   - - - - -   - - - - x -   - - - - - B   - - - - - B
x - - - x   - - - - W   - - - - W x   x - - - - W   B o - - - W
- - o - -   - - - - -   - - - o - -   - - o - - -   - - - - - -
- o W o -   - o W - -   - - W - - -   - - - W - -   - - - W - -
- - o - -   - - o - -   - - - - - -   - - - - - -   - - - - - -
x - - - x   x - - - -   W - - - - -   - W - - - -   - W - - - -
- - - - -   - - - - -   - - - - - -   - - - - - -   - - - - - -
Figure 1    Figure 2    Figure 3      Figure 4      Figure 5

Occupying an x-square results in Figure 2, which also shows the
remaining two o-squares that still need to be covered.
As before, the o-squares can't both be occupied, so at least one
needs to attacked.  The x-square is the only location available
that can attack without creating an over-covered square.

The rest of the figures show the result of occupying an x-square
from the previous figure using W for a white-squared knight or
B for a black-squared knight.  The x-squares are the only locations
that can cover an o-square without creating an over-covered square.

This process ends in Figure 5, which shows an o-square that can't be
covered from anywhere without creating an over-covered square.