Jonathan means that he will become a vampire too if Mina does.
Let's pick apart that quote you've highlighted:
To one thing I have made up my mind. If we find out that Mina must be a vampire in the end, then she shall not go into that unknown and terrible land alone.
Not that she shall not go into that unknown and terrible land (the state of vampirehood), but that she shall not go alone. He's saying it would be better for her to have company as a vampire than to suffer such a fate alone, and he's decided that if necessary he will make the supreme sacrifice to join her.
I suppose it is thus that in old times one vampire meant many.
One vampire "meant" many, in the sense that when one vampire is created, many more will follow on its heels. In this case, one vampire (Mina) would mean at least two (Mina and Jonathan). He's surmising that each time a human was converted to a vampire in the past, they would often be joined in that state by others who loved them and didn't want them to suffer eternally alone.
Just as their hideous bodies could only rest in sacred earth, so the holiest love was the recruiting sergeant for their ghastly ranks.
This confirms the above interpretation: love is the "recruiting sergeant" for the ranks of vampires, meaning it is love that causes new vampires to be created from humans. Love - in this case, Jonathan's love for Mina - causes one human to follow another into vampiredom, thus swelling the ranks of vampires more than if the first human 'turned' had nobody who loved them.