46 robots in total. It is recommended that you do the individual robot rushes from each Mega Man game first to get a general idea how to defeat the bosses. Watching the top leader board replays are very helpful. The following are the weaknesses to each boss:
1. Stone
2. Dust
3. Rain
4. Water
5. Needle
6. Fire
7. Air
8. Thunder
9. Top
10. G
11. Yamato
12. Dive
13. Blizzard
14. Hard
15. Shadow
16. G Hold
17 F Blast
18. Drill
19 N Bomb
20. Flash
21. Silver T
22. Magnet
23. Leaf
24. Skull
25. Crash / Flash
26. Gemini
27. Metal
28. Crystal
29. Ice
30. Snake
31. Spark
32. Pharaoh
33. Ring
34. Gyro
35. Bomb
36. Fire or Crash
37. Plant
38. Metal
39. Metal
40. Cut
41. Kick
42. Star
43. Bubble
44. Wind
45. Dust / Ring
46. Knight
I died probably every 3-4 bosses and still managed to get a silver.