Home Tiller

You have a classic.
As far as how much gear oil, if you need to add oil then it is very likely leaking out the shaft seals anyhow.
Typically, tip the tiller back nose up on the handlebars and fill to the top, it shouldn’t take much, ahlf a pint should be ample.
But before you do that, you should replace the seals, otherwise the oil leaks out into your garden soil, when it rains you will see the blue sheen in your garden from the oil in your soil.
Replace seals, fill and go.