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 $3$.