personally i'd buy 90 in your situation, well depending on the pricing, sometimes it is cheaper by 100 or 110 so then take the cheaper option
(kaihl bronze from CandyKeys where cheaper buying 1*100 than 9*10 when i bought mine for example)
switches rarely die without some help, like a spilled drink or a way too long soldering time, so you should be fine there.
although that only apply if you do not plan to disasemble the switches to mod them, then yeah take a few more, so that you have some lee way for lost parts and failures (then the number depend on how confident you are on your learning speed and organizational skills
, for me, being lazy and disorganized, i would get maybe 30 to 50 more, because i know spring will go pinging everywhere
)