A small coffee cup (of the sort that they often use in posh hotel and restaurants) typically holds about 6 fluid ounces, whereas a regular mug (or the sort that you might get at Fred's Burger Van) might hold roughly double that, at around 12 fluid ounces.
https://tinyurl.com/z6jecnpv
The Specialty Coffee Association of America suggests that 10g of coffee should be used for a 6 oz cup, meaning that the 90g in your question would provide you with 9 (small) cups of coffee or perhaps 4 or 5 mugs of it.
However some other sources suggest using only 7 to 9g of coffee for a 6oz cup, meaning that your 90g would then go a bit further than I've suggested above.