Netgear Germany gave an answer to exactly this question via Amazon a year ago: maximum 3 satellites.
I don't know if this has been extended via a firmware update in the meantime.
I have 2 satellites and have hardly any signal losses at a distance of approx. 18 metres (distance between router and first satellite approx. 9 metres and another approx. 9 metres to the second satellite, with several brick walls in between....). So even with 2 satellites there is still plenty of room for improvement, and even more so with 3...