You can use more analogue phones with one JabloPhone GDP-04Ai. PSTN extension of this JabloPhone supports the REN loading up to 3. REN stands for Ringer Equivalency Number. Standartd analogue phones have REN value of 1. That is why you can use up to 3 such analogue phones with one JabloPhone. Ask your vendor of phones about the REN number of your analogue phone.
If you use more analogue phones with one GDP-04Ai you need to buy a duplicator of analogue lines. Connect your GDP-04Ai to this duplicator by RJ-11 connector. Then connect each of your analogue phones with this duplicator.