How many charges are required to have an electric field

One charge is enough in order to have an electric field. 

In fact, every charge generates an electric field: for example, the electric field generated by a single point positive charge is radial, as shown in the attached figure. More complicate electric field configurations can be obtained adding charges or using more complicate charge distributions, but one charge is enough to have an electric field.

You will hear a hissing sound because of the carbon dioxide escaping the can or bottle