Limit to contribute depends on your age (and as noted above, USPS contributions have to be counted too).
Limit for individual for 2021 is $3600, if you ore 55 yo or older you can contribute another $1000.
If your partner is in your plan, you can contribute $7200, +$1000 if you are 55 yo or older. If your partner is also 55 yo or older, they can open their own HSA and contribute another $1000.
Unlike an IRA, you have to make these contributions by the end of the calendar year.
Even ignoring being able to reduce your taxable income (which I wouldn’t ignore because you might open up some Roth conversion space), you can invest most of your HSA and it grows tax-free.
Keep your medical receipts in order to support taking distributions from your HSA. You can save them and take the distribution in later years.