How many cats is too many?

I currently have a 13 year old (who ive had since she was born) and a 3 year old (rescue who was dumped at my place of work at 9 months old).

I work in a large outdoor property with lots of roads and forest nearby, and strays are not abnormal. We care for them as if they were our own and feed them daily, some of which we've done for 7+ years and counting. Anyway - we have been feeding a new cat for about 8 weeks and about a week ago we saw kittens emerge from the same sewer drain. We trapped them today (momma was too smart) and im considering taking them in myself. They're clearly feral and scared - based on size and eye color they are likely around 5 to 7 weeks old.

Im on the fence though, ive had people tell me i should and others that said i shouldnt. I live in a plenty large enough house and the cost of care/food/litter is not a concern for me personally but the most cats ive had at once is 2 so im not sure what to expect.

My old lady (13yo) has never been fond of other cats - she didnt like my old man before he passed away and she doesnt like my 3 yo right now (but he torments her trying to play). She doesnt really hiss or anything unless hes poking at her like a toddler. Theyll sleep in the same bed on opposite ends, share a litterbox etc without any issues.

What do you think i can expect if i choose to keep these two kittens (currently in my spare bathroom)? Is there a concern for additional stress on my old lady based on the above? Would love to have some opinions from the almighty reddit community!

Ultimately, i already have a foster lined up through a local rescue group should i choose not to go through with this. Am i being smitten by the kittens?