Add +f subtype 'p' (for 'paste'). So [2p]:15 means max 2 pastes per 15s.

This way you can limit the number of pastes going on in a channel, as
this is from everyone in that channel (like 'm') not individual (like 't').
If it is exceeded then we will simply reject the BATCH, similar to
how action d(rop) works for some other subtypes. You won't see the paste
on the channel, only the sending user receives an error (MULTILINE_PASTE_LIMIT).

Small note: a multiline BATCH of just 2 lines is not considered a paste.
We consider a multiline of 3+ lines as a paste. I think that is reasonable,
since a two-line-multiline is not that much of a paste ;).

In the default anti-flood profile (+F normal) we also set 2p per 15s,
so this means channels are by default limited to 2 pastes per 15s max.
Of course, you can override this with +f [4p]:15 or whatever you like.
In terms of +F profiles, the defaults are (maximum x pastes per 15 seconds):
very-strict: 1p
strict: 1p
normal: 2p
relaxed: 2p
very-relaxed: 3p

doc/conf/help/help.conf

@@ -403,11 +403,13 @@ help Chmodef {
 	"        k        Knock       +K";
 	"        m        Messages    +m               M";
 	"        n        Nickchange  +N";
+	"        p        Paste       drop             m, M";
 	"        t        Text        kick             b, d";
 	"        r        Repeat      kick             d, b";
 	" -";
 	" The difference between type m and t is that m is tallied for the entire";
 	" channel whereas t is tallied per user.";
+	" Type p counts multiline paste events (3+ lines) for the entire channel.";
 	" If you choose to specify an action for a mode, you may also specify a";
 	" time (in minutes) after which the specific action will be reversed.";
 	" See also https://www.unrealircd.org/docs/Channel_anti-flood_settings#Channel_mode_f";