A resource that shows how many ways an opening can be reached? Is there a resource (book/online) that shows how many ways an opening can be reached?
The point from chubbycantorset's answer about the futility of seeking the number of all ways to reach a position is a good one. Scid has a way of offering more limited (and more practically useful anyway) information than that. Its Opening Report tool allows you to input a position, and then generate a comprehensive statistical report covering games that reached that position (in whatever database you have open). Among the data presented is a tally of how many move-orders were used to reach the position in question that appeared in actual games from the database, as well as a list of the most popular.

For instance, in my big database of millions of games, when I generate a report for the standard Sicilian Scheveningen position

[fen "rnbqkb1r/pp3ppp/3ppn2/8/3NP3/2N5/PPP2PPP/R1BQKB1R w KQkq - 0 1"]


part of the information I receive is as follows, where the numbers in parentheses indicate how many games feature the move-order in question:

There were 82 move orders reaching this position. The top 8 are:
  1:  1. e4  c5 2. Nf3  e6 3. d4  cxd4 4. Nxd4  Nf6 5. Nc3  d6 (18538)
  2:  1. e4  c5 2. Nf3  d6 3. d4  cxd4 4. Nxd4  Nf6 5. Nc3  e6 (11028)
  3:  1. e4  c5 2. Nf3  d6 3. d4  Nf6 4. Nc3  cxd4 5. Nxd4  e6 (646)
  4:  1. e4  c5 2. Nf3  e6 3. Nc3  d6 4. d4  cxd4 5. Nxd4  Nf6 (597)
  5:  1. e4  c5 2. Nc3  e6 3. Nf3  d6 4. d4  cxd4 5. Nxd4  Nf6 (258)
  6:  1. e4  c5 2. Nf3  e6 3. d4  cxd4 4. Nxd4  d6 5. Nc3  Nf6 (144)
  7:  1. e4  e6 2. d4  c5 3. Nf3  cxd4 4. Nxd4  Nf6 5. Nc3  d6 (117)
  8:  1. e4  c5 2. Nf3  d6 3. Nc3  Nf6 4. d4  cxd4 5. Nxd4  e6 (92)