What are the guidelines on the seed pack? You should follow them for containers too.

If it were me, just 4 or 5 about 6" apart. Most types I grow tend to stay low and wide.
If too crowded, it leads to disease, lack of sun and fighting for food, regardless of where planted.

I hate thinning, so plant sparse. It's less painful...lol..

Have you grown it before? How are you planning to use it? Fresh or cooked? You may want to start a few more pots. If cooked , fresh spinach wilts down to nothing and takes a lot for a meal.

JoJo