As per the sizing document linked to by @Spooster the MX84 supports 100 tunnels and the MX100 supports 250.
This is the number of encrypted connections whether they be SD-WAN between MXs, or client VPN users.
If you don't have MX-MX links then the number simply covers the VPN client users. If you do have MX-MX links then you need to deduct their site to site links first.
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