The maximum (and unchangeable) particle limit is 4000, after which fireworks quality begins to degrade through fast expiration.
How low is this limit? A single firework containing 3 firework stars with trail effects is enough to *hover* at the limit.
In short: A single firework containing more than 3 trail effects, or more than 3 trail effects launched at a time begins wasting diamonds.
Allowing more particle control would be a nice feature.
Here's a mod demonstration which concludes with a very laggy 32k particle limit: http://www.youtube.com/watch?v=lGbA8kTGSZQ
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