Was curious to know what the most frequently appearing mutants are and, at least according to Marvel Database, it seems to be these. Accurate enough, no?:
3,409 appearances - Wolverine (1974)
2,215 appearances - Cyclops (1963)
2,146 appearances - Beast (1963)
1,882 appearances - Storm (1975)
1,707 appearances - Sub-Mariner (1939)
1,506 appearances - Iceman (1963)
1,435 appearances - Angel (1963)
1,373 appearances - Jean Grey (1963)
1,315 appearances - Colossus (1975)
1,308 appearances - Professor X (1963)
1,253 appearances - Nightcrawler (1975)
1,115 appearances - Kitty Pryde (1980)
1,068 appearances - Rogue (1981)
999 appearances - Emma Frost (1980)
934 appearances - Magneto (1963)
824 appearances - Psylocke (1976)
779 appearances - Cannonball (1982)
761 appearances - Gambit (1990)
728 appearances - Cable (1986)
623 appearances - Havok (1969)
589 appearances - Franklin Richards (1968)
577 appearances - Sunspot (1982)
559 appearances - Magik (1975)
554 appearances - Sabretooth (1977)
537 appearances - Rachel Grey (1981)
532 appearances - Jubilee (1989)
503 appearances - Polaris (1968)
497 appearances - X-23 (2004)
491 appearances - Wolfsbane (1982)
489 appearances - Bishop (1991)
487 appearances - Mystique (1978)
442 appearances - Domino (1992)
439 appearances - Moonstar (1982)
425 appearances - Dazzler (1980)
401 appearances - Warpath (1984)
397 appearances - Banshee (1967)
363 appearances - Boom-Boom (1985)
360 appearances - Firestar (1985)
360 appearances - Northstar (1979)
347 appearances - Forge (1984)
317 appearances - Rockslide (2003)
315 appearances - Justice (1975)
295 appearances - Namorita (1972)
288 appearances - Multiple Man (1975)
285 appearances - Siryn (1981)
283 appearances - M (1994)
280 appearances - Strong Guy (1985)
272 appearances - Pixie (2004)
268 appearances - Moira MacTaggert (1975)
266 appearances - Rictor (1987)
258 appearances - Karma (1980)
258 appearances - Magma (1983)
258 appearances - Meggan (1983)
254 appearances - Anole (2003)
252 appearances - Aurora (1979)
252 appearances - Toad (1964)
248 appearances - Hope Summers (2008)
246 appearances - Daken (2006)
236 appearances - Apocalypse (1986)
236 appearances - Husk (1986)
232 appearances - Blob (1964)
231 appearances - Chamber (1994)
223 appearances - Celeste Cuckoo (2001)
222 appearances - Mindee Cuckoo (2001)
217 appearances - Phoebe Cuckoo (2001)
214 appearances - Sebastion Shaw (1980)
212 appearances - Cypher (1984)
209 appearances - Armor (2004)
208 appearances - Sunfire (1970)
206 appearances - Madison Jeffries (1983)
199 appearances - Hellion (2003)
198 appearances - Quentin Quire (2003)
195 appearances - X-Man (1995)
190 appearances - Sage (1980)
180 appearances - Wiccan (2005)
178 appearances - Major Victory (1969)
174 appearances - Dr. Nemesis (1993)
172 appearances - Leech (1984)
168 appearances - Surge (2004)
166 appearances - Glob Herman (2001)
157 appearances - Mercury (2003)
155 appearances - Oya (2010)
150 appearances - Artie Maddicks (1986)
150 appearances - Dust (2002)
150 appearances - Fantomex (2002)
148 appearances - Namora (1947)
147 appearances - Madelyne Pryor (1983)
147 appearances - Wild Child (1983)
143 appearances - Abigail Brand (2004)
143 appearances - Genesis (2011)
142 appearances - Pete Wisdom (1995)
140 appearances - Marrow (1994)
138 appearances - Typhoid Mary (1988)
138 appearances - Whirlwind (1963)
136 appearances - Prodigy (2003)
133 appearances - Frenzy (1986)
132 appearances - Mastermind (1964)
131 appearances - Caliban (1981)
129 appearances - Blink (1995)
123 appearances - Pyro (1981)
121 appearances - Molly Hayes (2003)
120 appearances - Elixir (2003)
120 appearances - Legion (1985)
118 appearances - Callisto (1983)
117 appearances - Silhouette (1990)
117 appearances - Skin (1994)
115 appearances - Avalanche (1981)
115 appearances - Selene (1983)
115 appearances - Stryfe (1990)
114 appearances - Bling! (2005)
114 appearances - Layla Miller (2005)
114 appearances - Whizzer (1941)
108 appearances - Blindfold (2005)
108 appearances - Synch (1994)
107 appearances - Darkstar (1976)
105 appearances - Omega Red (1992)
100 appearances - Cecilia Reyes (1997)