What I believe happened, and the only thing that seems to add up to me, is that Voldemort aka Tom Riddle originally planned on making 6 horcruxes all along, and based all his magic around the number 7. He did not just incrementally add horcruxes as he went; he always planned from the get-go on splitting his soul into 7 parts.
There may have been an extra magical punch to this, or it may have been simply ego (7 being an important number worthy of the world's greatest wizard).
Or. most likely, it was the largest number of horcruxes possible. More would be too dangerous even for him to attempt. To this end, he made each horcrux with 1/7th of his soul, making them all equal, with the remaining 1/7 in his physical body.
Now, when he tried to kill Potter the child, and the spell backfired, the instability of his soul--having attempted magic never before performed and being in uncharted territory--caused it to spontaneously split in two, with half going into Harry. This essentially made Harry and Voldemort equals, Voldemort-soul-wise. Each had 1/14th in them.
Now, less than 1/7 of a soul still alive is just not enough soul to exist. Voldemort likely well knew this, and this was why he didn't make any more horcruxes, even when he discovered they were being destroyed.
In his original plan, he had already pushed the limits with 6, but had to be sure he could still survive with his 1/7th even if all 6 horcruxes were destroyed. Otherwise the extra horcruxes would be worthless, affording him no protection. However, he had no idea Harry now had half of that. This had thrown his whole original plan into chaos, and made him extremely vulnerable.
It would also explain why, when he used the kill curse on Harry in The Deathly Hallows, both of them temporarily left their bodies, and only returned together. Harry could kill Voldemort without this happening, because he had essentially a full soul in him, his own. He had enough soul to remain in the real world without Voldemort. Voldemort could not, because he only had 1/14 a soul left. Without the Harry-horcrux, there was not enough of an anchor left for him to remain present and alive.
The twist of all this would be that even if Voldemort had succeeded in killing Harry, he would have destroyed himself, as all the other horcruxes had been destroyed at that point. So essentially, Voldemort had lost when the snake was killed, and was then in deadly peril, alive only as long as Harry lived. He just didn't know it.