People in biblical times frequently came in the presence of dead bodies. So they must have needed a lot of the spring water mixed with red heifer ashes. But the Mishna says only 9 heifers were processed in history, with 1,000 years between the first two. So they must have highly diluted the ashes to accommodate everybody. (Halacha does not specify a density.) In fact, I read that much water was left over and used after the Second Temple was destroyed. Why didn't they keep diluting it to make it last indefinitely?
Rambam Parah Adumah 9:8 says that once the water has been mixed with the ashes, no further water can be added to further dilute the mixture. Once it has been made, an additional Parah Adumah is required to make more and add to the water. Thus, if any other water accidentally drops in, the entire mixture is pasul.
When even the smallest amount of other water – even water that was drawn for the sake of the ashes of the red heifer – becomes mixed with sanctified water, it is disqualified. Similarly, if dew descends into such water, it is disqualified. If other liquids or fruit juices fall into it, the entire quantity should be poured out. The container must be dried; only afterwards can other sanctified water be placed in it.
If ink, black earth, or dark earth, or any entity that leaves a mark falls into such water, it must be poured out, but there is no need to dry the container. The rationale is that if any portion of the entity that leaves a mark will remain, it will be apparent.
The Mishna in Para (3:5) states that there were 9 para adumahs in history. The Mishna states that Moshe (really Elazar) made a Parah Aduma and that Para Adumah lasted until Ezra which is about 800 years and then Ezra made a Parah Aduma when they returned from Bavel. Then, in the period of the second Beis Hamikdash they made an additional 7 para adumahs.
This means that for the first 800 years, they were able to use the water that Moshe made and keep putting the remainder (aside from the few drops sprinkled) back in the container. This is like the anointing oil used for the Kohen gadol that Moshe made. From Ezra to the destruction of the second temple, they used 8 such cows.
It appears that either they were unable to keep the water completely pure during the second temple, or they decided to make more as they found a cow that could be used for Parah Adumah because of the fear that they might run out. Seven in 420 years is once every 60 years, so it might be once a yovel.