With $5$ you can using the following moves:

.....  .....  .....  .....  .....  .....  .....  ..oo.  ...oo 
.....  ..o..  ..o..  ..o..  ..oo.  ...oo  ..ooo  ..ooo  ..ooo 
.oo..  .oo..  .oo..  ..oo.  ..oo.  ..oo.  ..oo.  .....  ..... 
ooo..  oo...  .oo..  ..oo.  ..o..  ..o..  .....  .....  ..... 

So the only number of pieces for which no configuration can go to infinity is $3$.

With $5$ you can using the following moves:

.....  .....  .....  .....  .....  .....  .....  ..oo.  ...oo 
.....  ..o..  ..o..  ..o..  ..oo.  ...oo  ..ooo  ..ooo  ..ooo 
.oo..  .oo..  .oo..  ..oo.  ..oo.  ..oo.  ..oo.  .....  ..... 
ooo..  oo...  .oo..  ..oo.  ..o..  ..o..  .....  .....  ..... 

So the only numbers of pieces for which no configuration can go to infinity are $1$ and $3$.

With $5$ you can using the following moves:

...
 ...
oo.
ooo

...
oo.
 oo.
..o

 ...
 oo.
 .oo
 ..o

...
 ooo
.oo
 ...

 .oo
ooo
...
 ...

 oo.
ooo
 ...
 ...

So the only number of pieces which can't go to infinity is $3$.

With $5$ you can using the following moves:

.....  .....  .....  .....  .....  .....  .....  ..oo.  ...oo 
.....  ..o..  ..o..  ..o..  ..oo.  ...oo  ..ooo  ..ooo  ..ooo 
.oo..  .oo..  .oo..  ..oo.  ..oo.  ..oo.  ..oo.  .....  ..... 
ooo..  oo...  .oo..  ..oo.  ..o..  ..o..  .....  .....  ..... 

So the only number of pieces for which no configuration can go to infinity is $3$.