In S07E04 Tyrion says
We still have enough ships to carry the Dothraki to the mainland.
(about the 26 minutes mark)
That's all we know for sure.
According to Tyrion's attack plan mentioned in S07E02 (around the 23 minute mark), only the Iron Fleet is travelling to Dorne at the moment it's attacked and destroyed by Euron (we also only see the Ironborn fighting).
In S07E03 (around the 51 minute mark) Varys says
Your Grace, he's already destroyed a good portion of our fleet. To send our remaining ships after him..."
and this is after the Unsullied left for Casterly Rock - as Varys says
They will be there soon
just a minute later in this same episode - but before Varys learns they were intercepted there by Euron's ship.
This means Dany still has a portion of the fleet anchored off Dragonstone - these would be the "remaining ships" Varys mentions. After all, they can't risk the ships destroyed by Euron or the ones sent to Casterly Rock, so he must be talking about some other ships, ones that are available to them at that moment. These are most likely Dany's own ships, the ones she took from the Slave Masters in Mereen. These are the ones she uses to take the Dothraki to the mainland in S07E5.