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 $3$.