there isn't really a limit to how much beatmaps you can have on GRAVEYARD, however there's a limit you can have on PENDING, which goes as follows:
"you can have a maximum of 4 maps + 1 for every ranked map you have (caps at 2). with supporter, you can have a maximum of 8 maps + 1 for every ranked (caps at 12)."
if you reach this PENDING limit, you won't be able to submit maps until one of the maps goes to GRAVEYARD