Do you want it so when someone joins a channel:
Users in chan | Chan Limit
1 | 3
2 | 4
3 | 5
4 | 6
5 | 7
6 | 8
7 | 9
8 | 10
9 | 11
10 | 12
11 | 13
12 | 14
so the channel limit is the number of users + 2
or:
Users in chan | Old Limit | When ONE person joins,
| would be | New Channel Limit
1 | 3 | 5
2 | 4 | 7
3 | 5 | 9
4 | 6 | 11
5 | 7 | 13
6 | 8 | 15
7 | 9 | 17
8 | 10 | 19
9 | 11 | 21
10 | 12 | 23
11 | 13 | 25
12 | 14 | 27
you see the difference?
if you want the second, then use what Collective said.
If you want the first one, where there is space for 2 more people to join (like an eggdrop bot), use:
mode # +l $calc($nick($chan,0) + 2)
on @*:JOIN:#ChannelName:{
mode # +l $calc($nick($chan,0) + 2)
}