Hello,
1) BES uses one MAPI client per BES server. In your case, where you have 10 BES servers, you have 10 MAPI clients operating.
2) NSPI is used by the MAPI client to retrieve information from the directory. Without an NSPI connection, the MAPI client will not function and BES will stop working properly.
3) Assuming your 10,000 users are spread evenly over the 10 BES servers, you would need to set the reg key to 10,000 (1000 connections per server X 10 servers)
4) Yes, you can use multiple BES service accounts but it is a one-to-one ratio of service account to server. In your case, you could use 10 different service accounts (1 per server). In that case, you could set the reg key to 1000. Here is information on how you change the service account -
http://www.blackberry.com/btsc/viewContent.do?externalId=KB04293&sliceId=1JJ