I initially thought their numbers were referring to the order of planets in the solar system (since Saturn is 5 and Venus is 2) but that quickly got debunked when the anime revealed Jupiter is 1.
On the other hand, their numbers
perfectly correspond to which chair they sit at in Marijoa.
From left to right:
1- Jupiter 2- Venus 3- Topman 4- Mars 5- Saturn
And likewise, when Oda paneled them together, he had the same order of their chair positions too:
So yeah, Garling will be number 5 since he sits at Saturn’s chair now.