Usually I get class sizes between 18 and 26 students, and this year I expect about 20-22. To be safe, I keep 24 desks; I also like the symmetry of that number, and the variety of desk configurations. Pairs, trios, quads, and groups of six are all possible here. (This middle-school teacher avoids using the word 'sextet,' even in the proper context.)
I inherited several bookshelves, but I ditched the worst one last summer. The rest are shoved up against the walls to give the most possible space for student desk space. I also inherited the chalkboard timeline that you can see below. It's probably the best use of a chalkboard. I hate writing with chalk!
I believe it is important to keep a space for the teaching assistants who spend 1 or 2 class periods in my room. They need a regular place to keep their belongings, and a strategically-chosen spot to observe students on their caseload.
I'm fortunate to have a window nook that's just the right place for my teacher desk. It's a great place to watch the sun setting at 3:45 in December.... It's also a good spot to hide for one period per day, when I'm sharing this room with a language teacher (last year Spanish, this year it's French). This reminds me of my earlier teaching days when I didn't have my own classroom. I can't complain too much about furniture, bulletin boards, outlets, etc. because I feel fortunate to have a room at all!