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 $3$.
Saúl RM
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