I just did some testing (your question made me curious) and I cannot see there is a hard limit. The limit would appear to be as many as you can fit into the character limit of 4000.

253 nested IFs definitely works:

=IF(a@row = 1, IF(b@row = 1, IF(c@row = 1, IF(d@row = 1, IF(e@row = 1, IF(f@row = 1, IF(g@row = 1, IF(h@row = 1, IF(i@row = 1, IF(j@row = 1, IF(k@row = 1, IF(l@row = 1, IF(m@row = 1, IF(n@row = 1, IF(o@row = 1, IF(p@row = 1, IF(q@row = 1, IF(r@row = 1, IF(s@row = 1, IF(t@row = 1, IF(u@row = 1, IF(v@row = 1, IF(w@row = 1, IF(x@row = 1, IF(y@row = 1, IF(z@row = 1, IF(aa@row = 1, IF(ab@row = 1, IF(ac@row = 1, IF(ad@row = 1, IF(ae@row = 1, IF(af@row = 1, IF(ag@row = 1, IF(ah@row = 1, IF(ai@row = 1, IF(aj@row = 1, IF(ak@row = 1, IF(al@row = 1, IF(am@row = 1, IF(an@row = 1, IF(ao@row = 1, IF(ap@row = 1, IF(aq@row = 1, IF(ar@row = 1, IF(as@row = 1, IF(at@row = 1, IF(au@row = 1, IF(av@row = 1, IF(aw@row = 1, IF(ax@row = 1, IF(ay@row = 1, IF(az@row = 1, IF(ba@row = 1, IF(bb@row = 1, IF(bc@row = 1, IF(bd@row = 1, IF(be@row = 1, IF(bf@row = 1, IF(bg@row = 1, IF(bh@row = 1, IF(bi@row = 1, IF(bj@row = 1, IF(bk@row = 1, IF(bl@row = 1, IF(bm@row = 1, IF(bn@row = 1, IF(bo@row = 1, IF(bp@row = 1, IF(bq@row = 1, IF(br@row = 1, IF(bs@row = 1, IF(bt@row = 1, IF(bu@row = 1, IF(bv@row = 1, IF(bw@row = 1, IF(bx@row = 1, IF(by@row = 1, IF(bz@row = 1, IF(ca@row = 1, IF(cb@row = 1, IF(cc@row = 1, IF(cd@row = 1, IF(ce@row = 1, IF(cf@row = 1, IF(cg@row = 1, IF(ch@row = 1, IF(ci@row = 1, IF(cj@row = 1, IF(ck@row = 1, IF(cl@row = 1, IF(cm@row = 1, IF(cn@row = 1, IF(co@row = 1, IF(cp@row = 1, IF(cq@row = 1, IF(cr@row = 1, IF(cs@row = 1, IF(ct@row = 1, IF(cu@row = 1, IF(cv@row = 1, IF(cw@row = 1, IF(cx@row = 1, IF(cy@row = 1, IF(cz@row = 1, IF(da@row = 1, IF(db@row = 1, IF(dc@row = 1, IF(dd@row = 1, IF(de@row = 1, IF(df@row = 1, IF(dg@row = 1, IF(dh@row = 1, IF(di@row = 1, IF(dj@row = 1, IF(dk@row = 1, IF(dl@row = 1, IF(dm@row = 1, IF(dn@row = 1, IF(do@row = 1, IF(dp@row = 1, IF(dq@row = 1, IF(dr@row = 1, IF(ds@row = 1, IF(dt@row = 1, IF(du@row = 1, IF(dv@row = 1, IF(dw@row = 1, IF(dx@row = 1, IF(dy@row = 1, IF(dz@row = 1, IF(ea@row = 1, IF(eb@row = 1, IF(ec@row = 1, IF(ed@row = 1, IF(ee@row = 1, IF(ef@row = 1, IF(eg@row = 1, IF(eh@row = 1, IF(ei@row = 1, IF(ej@row = 1, IF(ek@row = 1, IF(el@row = 1, IF(em@row = 1, IF(en@row = 1, IF(eo@row = 1, IF(ep@row = 1, IF(eq@row = 1, IF(er@row = 1, IF(es@row = 1, IF(et@row = 1, IF(eu@row = 1, IF(ev@row = 1, IF(ew@row = 1, IF(ex@row = 1, IF(ey@row = 1, IF(ez@row = 1, IF(ga@row = 1, IF(gb@row = 1, IF(gc@row = 1, IF(gd@row = 1, IF(ge@row = 1, IF(gf@row = 1, IF(gg@row = 1, IF(gh@row = 1, IF(gi@row = 1, IF(gj@row = 1, IF(gk@row = 1, IF(gl@row = 1, IF(gm@row = 1, IF(gn@row = 1, IF(go@row = 1, IF(gp@row = 1, IF(gq@row = 1, IF(gr@row = 1, IF(gs@row = 1, IF(gt@row = 1, IF(gu@row = 1, IF(gv@row = 1, IF(gw@row = 1, IF(gx@row = 1, IF(gy@row = 1, IF(gz@row = 1, IF(ha@row = 1, IF(hb@row = 1, IF(hc@row = 1, IF(hd@row = 1, IF(he@row = 1, IF(hf@row = 1, IF(hg@row = 1, IF(hh@row = 1, IF(hi@row = 1, IF(hj@row = 1, IF(hk@row = 1, IF(hl@row = 1, IF(hm@row = 1, IF(hn@row = 1, IF(ho@row = 1, IF(hp@row = 1, IF(hq@row = 1, IF(hr@row = 1, IF(hs@row = 1, IF(ht@row = 1, IF(hu@row = 1, IF(hv@row = 1, IF(hw@row = 1, IF(hx@row = 1, IF(hy@row = 1, IF(hz@row = 1, IF(ja@row = 1, IF(jb@row = 1, IF(jc@row = 1, IF(jd@row = 1, IF(je@row = 1, IF(jf@row = 1, IF(jg@row = 1, IF(jh@row = 1, IF(ji@row = 1, IF(jj@row = 1, IF(jk@row = 1, IF(jl@row = 1, IF(jm@row = 1, IF(jn@row = 1, IF(jo@row = 1, IF(jp@row = 1, IF(jq@row = 1, IF(jr@row = 1, IF(js@row = 1, IF(jt@row = 1, IF(ju@row = 1, IF(jv@row = 1, IF(jw@row = 1, IF(jx@row = 1, IF(jy@row = 1, IF(jz@row = 1, IF(ka@row = 1, IF(kb@row = 1, IF(kc@row = 1, IF(kd@row = 1, IF(ke@row = 1, IF(kf@row = 1, IF(kg@row = 1, IF(kh@row = 1, IF(ki@row = 1, IF(kj@row = 1, IF(kk@row = 1, IF(kl@row = 1, IF(km@row = 1, IF(kn@row = 1, IF(ko@row = 1, IF(kp@row = 1, IF(kq@row = 1, IF(kr@row = 1, IF(ks@row = 1, 1, 0)))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))