Es sei denn is a fixed phrase that does not influence what follows; especially, there is no need for it to be followed by the present subjunctive. Note that sind isn't present subjunctive, but indicative; the former would be seien.
The question remains why singular es is followed by plural sind. This happens when es is the subject and a plural noun phrase is the subject complement.
Schau dir mal die Fotos an. Es (subject) sind einige meiner besten Aufnahmen (subject complement).
Note that the grammatical structure changes completely in cases such as the following.
Schau dir mal die Fotos an. Es (expletive) sind einige meiner besten Aufnahmen (subject) darunter (subject complement).
The expletive es has no grammatical function and disappears when the sentence is rearranged.
Schau dir mal die Fotos an. Einige meiner besten Aufnahmen sind darunter.