Ciao Joep,
There are hotels in Florence that do offer that amount of rooms, but it basically means a large portion of the hotel so booking has to be done WELL in ADVANCE to find all the rooms you need available.
You don't mention WHEN your retreat is planned, just that it is annual, so I assume some time this year. As long as it doesn't occur when Pitti Immagine organizes major trade fairs, you might be able to find availability.
If places are no longer available in Florence for the dates you need, you will have to consider looking beyond Florence center and at that point it adds to the logistics since you'll have to plan transport to and from the city center, if you're planning on including the center's attractions as part of your itinerary. How long is the retreat for?
So not knowing more, I can give you and idea of top hotels in Florence with the amount of rooms that can host your retreat but you will have to contact them individually to ask for availability of the rooms, as well as adding on the cost of the meeting rooms and lunch (breakfast is always included in hotels, lunch would be the extra which can be planned on location if they have a restaurant or catered):
- Grand Hotel Mediterraneo - 327 rooms + lots of meeting rooms with various capacities - located on Lungarno del Tempio just outside the city center, opposite end of the train station. The new tram line will be passing right in front so at the moment there is a bit of construction going on in front of it.
- Grand Hotel Adriatico - 129 rooms and many meeting rooms to host your group - on Via Maso Finiguerra so closer to the train station.
- Starhotels Michelangelo - has 119 rooms and 7 meeting rooms for up to over 300 people, so this could comfortably meet your group's needs.
- Sina Villa Medici is on via il Prato, really close to the SMN train station. With 100 rooms and 6 meeting rooms to host up to 100 people, it is the smallest hotel that just might be able to host your retreat that is also very centrally located.
These hotels are also in the center, have the number of hotel rooms but small meeting rooms -- for these you could consider a separate meeting hall depending on the space you need:
- The Hoxton Florence - a relatively newer hotel, close to Piazza della Libertà. While it does have 161 rooms, there is only one meeting room with capacity of up to 60 people so it might be small for you from that point of view.
- Grand Hotel Minerva in Piazza Santa Maria Novella has 97 rooms and 4 meeting rooms, max capacity for the meeting rooms also small at around 70 people.
- Hotel MH Florence & Spa has 82 rooms and 3 meeting rooms for up to 85 people, but being right along the train station makes it very central and you could external spaces for the meetings.
- 25Hours Hotel Piazza San Paolino - with 170 rooms, it has 3 meeting rooms for capacity of up to 80 people. It is in the SMN church area, right below the Novecento museum, so also very central.
Going outside of the historical center but not too far:
- the Social Hub Florence Lavagnini has 166 rooms and 5 meeting rooms with capacities of up to 200 people. It is right in front of the Fortezza da Basso so very close to the historical center.
- the Social Hub Florence Belfiore has 248 rooms -- these two locations are particular, they offer hotel rooms as well as student housing and large community spaces. They also have restaurants and meeting spaces so since your retreat is a "creative" retreat, these might be something you would like to consider in particular. It is a different sort of setting that the usual hotel. Both are just outside the avenues that surround the historical center so they are still very close to all of the attractions and it's easy to walk into the center.
- Hotel Raffaello has 140 rooms and 2 meeting rooms for up to 100 people. While it is further out of the center, it is right along the T1 tram line that connects the center with the Careggi hospital so easy to get to the center.
- The Doubletree by Hilton Florence Metropole has 212 rooms and 10 meeting rooms, so enough capacity for your group. It is close to the Viadotto del Ponte all'Indiano on the western side of town, near the FI-PI-LI roadway so easy to get to if you're arriving in Florence with buses.
- Hotel Grifone has 86 rooms and 1 large meeting room for up to 100 people, it is slightly outside of the city center in the Bellariva area east of the city center. It is a good 30 min walking distance to the center but there are buses that go directly to SMN train station.
- The Hilton Garden Inn Florence Novoli is in the San Donato/Novoli area and easy to get to and tram with the tram, either to the airport or to the center. It has 121 rooms and 2 meeting rooms, for capacity of up to 150 people.
I had to some research myself for hotels with this amount of rooms because I knew of some but not all -- it's interesting to see how many exist right under my nose that I didn't realize had such capacity in terms of rooms! It makes sense, Florence is a popular destination for both conferences and trade fairs and meetings in general, but precisely for that reason, even with these I hope you are still in time to book for when you want to come to Florence.
As I said, you'll have to contact them directly to ask for availability and more info on costs and more.
If these turn out to not work out, let me know and I can help look for more hotels, even a bit further out of the center along the towns connected to Florence by train.