In theory 50.. https://www.apple.com/au/airport-express/specs/
In reality.. about 10 per unit.. but you might stretch it to 20 if you use both bands carefully. Remember you only have 100mbit ethernet connection.. at absolute max you are going to reduce 10 clients down to 10mbit.. and 20 clients to 5mbit.. that is very slow when you start moving files. And wireless just ain't that efficient.. so much less is better.
You might also find running 10 units harder than you might think unless they are 100M apart.. wireless provides 3 non-overlapping channels at 2.4ghz and about 3 times that number at 5ghz depending on location in the world.
If they are too close to each other.. you will have chaos.
Airport Express is a home unit.. for office you should buy pro grade gear.