Somewhere between a few and several.
You're biggest hit will be surfaces so you need to keep those down to a minimum.
Vertex manipulation slows things down.
Bones slow things down.
Fill rate (how much screen is being drawn) is a frame rate killer.
Alpha blend slows things down.
Multiply blend slows things down.
Additive blend slows things down.
Also combine these 3 blend modes with your fill rate for really impressive frame rate hits.
The list goes on... My point is that there is no difinitive answer to your question.
You're biggest hit will be surfaces so you need to keep those down to a minimum.
Vertex manipulation slows things down.
Bones slow things down.
Fill rate (how much screen is being drawn) is a frame rate killer.
Alpha blend slows things down.
Multiply blend slows things down.
Additive blend slows things down.
Also combine these 3 blend modes with your fill rate for really impressive frame rate hits.
The list goes on... My point is that there is no difinitive answer to your question.