I see the problem : 'every' = 'all' or 'each' but neither term applies if there is only one. The Latin is : Omnis substantia est necessario infinita. On the surface it would have been better, more intelligible, if Spinoza had said something like 'Substance as such is infinite', which would be perfectly consistent with there being only one substance.
Something definitely needs explanation. There are two main possibilities :
1 Spinoza has simply made a logical mistake
2 Spinoza is moving from one sense of substance to another and P8 is caught in the transition.
It is perfectly possible that Spinoza simply has made a logical mistake but I prefer the second explanation, which I think more likely.
SUBSTANCE : SHIFT OF SENSE
A plurality of substances was a perfectly standard idea in Aristotle. Very roughly a substance (ousia) was whatever qualities, quantities, and the other categories inhered in as their support. There could not, for instance, be a quality unless it was a quality of a substance; it could not exist on its own but needed to be a quality of something, namely a substance. Aristotle assumed that there were indefinitely many substances. Descartes held that there was ultimately and strictly only one substance, namely God, but granted de facto substance-status to mind and matter.
This is the working idea from which Spinoza starts. There was general agreement (though Locke had his difficulties) that substances exist. Hence he is ready to talk of 'two substances' in I. P2. But the assumption of a plurality of substances is only a concession to the standard way of talking. There is a pointer to his own view of the single substance in I.P5 but he is still working with the standard view in I. P8.
Then in Scholium [Note] 2 to I. P8 he reveals his hand, develops his own distinctive position : 'there exists only a unique substance of the same nature' (Spinoza, Ethics, tr. G.H.R. Parkinson, Cambridge : CUP, 2000, 80) - substantiae existentiam ... eiusdem naturae. So how do we get to this point ?
We need to go back to I.P7 : 'It belongs to the nature of substance to exist' (Parkinson, 78). Its existence follows from its nature, and that nature is to be self-dependent, self-contained; to be in Spinoza's language 'that which is in itself' (I, Def, 3 : quod in se est; Parkinson, 75). It cannot be self-dependent and self-contained if there is another substance which can act as an external cause or limitation. Hence 'there exists only a unique substance of the same nature'.
'Of the same nature' : an enigmatic little addendum. I think what he means is that a substance has or supports attributes. There is, Spinoza thinks, an infinity of attributes I. Prop. 11; Parkinson, 82) of which we are acquainted with only two, namely thought and extension (II, Axiom 5; Parkinson, 114). 'Of the same nature' means that there could not be two substances having or supporting the same attributes (attributes of the same nature), else one substance could act as an external cause or limitation in regard to the other in respect of those attributes. But since Spinoza's substance has an infinity of attributes, i.e. all the attributes there are, the existence of a second - any second - substance is impossible on pain of its acting an an external cause or limitation.
On the matter of substance's being infinite, I think infinity follows from the infinity of its attributes, and (more basically) from the one substance's nature as self-contained and unlimited by any other substance. This makes it all-inclusive, hence in one sense infinite.
It's no easy task to make full sense of Spinoza. I have simply offered the best I can.
Spinoza, Ethics, tr. G.H.R. Parkinson, Cambridge : CUP, 2000. See Introduction, 55-7.
Joel I. Friedman, 'An Overview of Spinoza's "Ethics"', Synthese, Vol. 37, No. 1, Spinoza in Modern Dress (Jan., 1978), pp. 67-106.
William Charlton, 'Spinoza's Monism', The Philosophical Review, Vol. 90, No. 4 (Oct., 1981), pp. 503-529.
Stuart Hampshire, Spinoza, London : Penguin, 2007.