Is there a limit on the number of notebooks?
From what I can tell, the only real limit is dependent on your syncing service and target. You'll definitely need to make sure you have enough space for storing everything and some webdav services have syncing limits. You'll need to do some research and due diligence on your end. There's plenty of posts here about various different services with Nextcloud being hosted on your own private server or external vps being pretty much the de facto way to go. 
I’m also unable to create another notebook. I have currently 70+ (I think 74) and can’t create another one.
██████████████████ ████████ sam@shxps
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██████████████████ ████████ OS: Manjaro Linux x86_64
██████████████████ ████████ Host: XPS 8700
████████ ████████ Kernel: 5.6.19-2-MANJARO
████████ ████████ ████████ Uptime: 21 days, 21 hours, 40 mins
████████ ████████ ████████ Packages: 1269 (pacman)
████████ ████████ ████████ Shell: zsh 5.8
████████ ████████ ████████ Resolution: 1920x1080
████████ ████████ ████████ DE: GNOME 3.36.4
████████ ████████ ████████ WM: Mutter
████████ ████████ ████████ WM Theme: Matcha-sea
████████ ████████ ████████ Theme: Matcha-sea [GTK2/3]
████████ ████████ ████████ Icons: Papirus-Dark-Maia [GTK2/3]
Terminal: gnome-terminal
CPU: Intel i7-4790 (8) @ 4.000GHz
GPU: NVIDIA GeForce GTX 745
Memory: 8661MiB / 15966MiB
Joplin 1.0.227 (prod, linux)
Client ID: *****
Sync Version: 1
Profile Version: 30
Keychain Supported: No
Revision: db77170 (master)