I think it's 300 exactly. the reason for the limit like I've said before is probably stability. there's a chart somewhere regarding how fast the 3ds reads and the home menu stability. in fact, Nintendo themselves recommend no greater than 32GBs. that is according to their official site. beyond that takes longer and longer to load. it's unlikely 300 titles will fit on 32GBs, so they deemed that mandate necessary.
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nm, I think it's 100. the wii u is 300, unless I'm wrong about that and got them confused. the wii u seems to include default apps like system settings in that number which is y it's like 292 in reality. I'm not sure if the 3ds is the same, but I have gone over the limit before requiring deletion. games would appear in the deletion screen, but the games would overwrite other games if installed.