How many proteins are in the Earth's proteome?
Current records
According to Uniprot, there are 85,381,808 protein records, and with the UniRef90 filter (i.e removing records that can be represented by an entry with at least 90% sequence similarity), there are 42,424,511. However, these databases are moving targets and will change over time. We will sequence more species, find novel splice isoforms, and various other methods will expand the databases. Indeed, the databases will be truncated also from time to time as some hypothetical proteins may be based on genes that turn out to not code for proteins after all.
In 2007, a study estimated that the earth's proteome would contain around 5 million sequences, and that the majority of these would be elucidated by 2012. I suspect this is a very thorough study, however, a lot has changed in the last 10 years. This estimate is actually less than the nearly 9 million species estimated in more recent studies.
Approximate estimate
So let's do some back of the envelope maths. Let's assume the article estimating nearly 9 million species is about right, and that we've only catalogued some 1.2 million. But UniProt isn't even close to this number.UniProt contains 25477 scientific names in it's controlled vocabulary. So for 25 thousand names, we have 85 million protein records. What if we had 8.75 million names?
Let's assume:
$\frac{Predicted~Proteins}{Predicted~Species}=\frac{Known~Proteins}{Known~Species}$
We can rearrange this to:
$\frac{Predicted~Species~\times~{Known~Proteins}}{Known~Species}= Predicted~Proteins$
Generous estimate (Uniprot, 335527 proteins per species):
$\frac{8750000\times{85381808}}{25477}=2.932413e^{+10}$
Conservative estimate (Swissprot, 41 proteins per species):
$\frac{8750000\times{554241}}{13408}=3.616952e^{+7}$
For the sake of completeness let's assume the number of &lt;90% identical proteins will remain around half that value. We can say that there might be around $1.8e^{+7}$ to $1.5{e}^{+10}$ &quot;unique&quot; proteins, less than a trillion ($1e^{+12}$). Given the absurdly generous 335 thousand proteins and very stingy 41 proteins per species, we can be fairly certain that if there are indeed 8.75 million species, the number of proteins will fall between those estimates.
The biggest assumption here is that the proteins have a linear relationship with species which is unlikely to be the case, and at the generous estimate we are pretending that there are no proteins in UniProt that don't have species annotation. As for Swissprot, this only includes proteins that have been manually curated, so this ignores many proteins that are safe to assume exist and typically only covers proteins that are of interest to scientists.

A minor correction to your question, the UniProt lists ~20 thousand protein coding genes in the human proteome, not millions. Those protein coding genes are subject to various post translational modifications and isoform splicing, so there will be more final proteins than 20k.