I was trying to update the DWM and got this error message from emerge:
dwm.c: In function 'createmon':
dwm.c:654:15: error: 'nmaster' undeclared (first use in this function)
dwm.c:654:15: note: each undeclared identifier is reported only once for each function it appears in
dwm.c: At top level:
dwm.c:1030:1: warning: 'incnmaster' defined but not used
make: *** [dwm.o] Error 1
It seems that dwm 6.0 has included the code of nmaster patch, or a re-implementation. Anyhow, you need to update your config.h for nmaster variable, even you don't use nmaster.
/* layout(s) */
// snip
static const int nmaster      = 1;    /* number of clients in master area */
// snip

static Key keys[] = {
// snip
 { MODKEY,                       XK_i,      incnmaster,     {.i = +1 } },
 { MODKEY,                       XK_d,      incnmaster,     {.i = -1 } },
// snip
The incnmaster function is to change the amount of clients in master area. I bind them to the keys which fits into my configuration, here is the diff I updated for this upgrade.

Before 6.0, I hadn't used nmaster patch, but I did feel the need of similar feature. There were some moments, master area is, well, too big for just one client. However, I have gridmode patch, sometimes it can sort of have the layout I need.