For ActiveSync search command, Mailbox (email search): 100 limit. Search will return the top 100 entries. It also returns the total match. You can then refine your filter to get the relevant data into the top 100 entries. GAL search (to find contacts/recipients in GAL): up to 15 entries.

For Compose Email recipient search, less than 3 characters: Combine data from Local Contacts DB + data from GCSCRecipient table in DB. More than 3 chars: Use GAL Search (see above).