Your mistake is understandable in what you expect should be the syntax.
Each account has it's own mailbox named "INBOX"
In Mail the mailbox inbox is the reference to the top level inbox that shows contents of all other inboxes named "INBOX"
2 examples:
Example 1
tell application "Mail"
set inboxes to first mailbox of every account whose name is "INBOX"
set messageCount to 0
repeat with i from 1 to number of items in inboxes
set this_item to item i of inboxes
if this_item is not missing value then
set thisCount to (count of (messages of this_item))
set messageCount to thisCount + messageCount
log thisCount
end if
end repeat
end tell
log messageCount
Example 2
tell application "Mail"
set messageCount to (count of (messages of inbox))
end tell
log messageCount
Both return and log the same total.
But example 1 also logs the individual count of each "INBOX"
A good place to start is to read through: AppleScript Fundamentals