What are the max number of allowable parameters per database provider type?

Oracle: 64,000. Source

MySQL:

• By default, there is no limit. The MySQL “text protocol” requires that the .NET client library substitute all parameters before sending the command text to the server; there is no server-side limit that can be enforced, and the client has no limit (other than available memory).
• If using “prepared statements” by calling MySqlCommand.Prepare() (and specifying IgnorePrepare=false in the connection string), then there is a limit of 65,535 parameters (because num_params has to fit in two bytes).

PostgreSql: EDIT: 34464 for a query and 100 for a function as per Magnus Hagander’s answer (Answer copied here to provide a single point of reference)

SqlLite: 999 (SQLITE_MAX_VARIABLE_NUMBER, which defaults to 999, but can be lowered at runtime) – And for functions default is 100 parameters. See section 9 Of Run-time limits documentation