Q:

How many houses can I buy?

So far I have found a purchased two houses, one near the festival and one on the coast.
How many purchasable properties are available for me to buy?

A:

There are multiple houses / residents to buy. 
From what I can find on google there are about 12 houses/castles.
Here is the full list as acquired from this site:

Edinburgh Castle:
Located in the heart of the city, Edinburgh Castle offers the best of Horizon's properties
15,000,000 CR.
Bamburgh Castle:
This historical home overlooks Forza Horizon 4's coast, situated alongside the long east-facing beach.
10,000,000 CR.
Lake Lodge:
If castles aren't for you, Lake Lodge is a top-tier lakeside house, overlooking Derwentwater.
5,000,000 CR. (Free for VIP Pass owners)
Fairlawn Manor:
Fairlawn Manor is a bold stately home located in the map center, positioned among the upper-class Forza Horizon 4 homes
2,000,000 CR.
Derwent Mansion:
Found on the west side of Derwentwater, this lakeside property is a cheaper alternative to Lake Lodge.
1,500,000 CR.
Castleview Road:
Found in the southern suburbs of Edinburgh, this offers just a quick drive to the city center
750,000 CR.
The Huntsman Lodge:
Hidden deep into Lakehurst Forest, this house keeps you far away from the busy roads of Horizon
750,000 CR.
Thatch Corner:
This cozy beachside thatched cottage is another quiet retreat from the action.
500,000 CR.
Kingfisher Cottage:
Located beside the Derwentwater reservoir, this historical cottage is tucked just off the roadside.
350,000 CR.
Croftdale Farm:
Take in the Scottish Highlands with this sky-high property, hidden near Glen Rannoch in the north-west corner of the map.
200,000 CR.
Sunflower Meadows:
This white semi-detached cottage is found in Ambleside, offering another excellent option for first-time buyers
200,000 CR.
The Gables:
In an unlikely turn of events, The Gables is a property gifted by film makers after your work as a movie stunt driver.
Best of all, this location is entirely free.

