This of course varies between 1v1, 2v2, 3v3 and 4v4 maps, but as sirpsychoj said, complex models cause more work than simple models.
What is a healthy number of objects a map can have? Even just an approximate figure would be good.
For example Angoville has 701 objects, 847 splats and 653 splines. Rails and Metal has 3116 objects, 3216 splats and 871 splines. Montargis region has 3983 objects, 5087 splats and 2053 splines.
Decals are like splats, 2d textures added to the terrain. In mapping contests hosted by Relic decals were unallowed to use in maps.
From my forum search I came across a thread saying decals are bad for performance. What are decals?
Map can have over 2000 splines like Montargis Region has, but still runs pretty smooth, but of course with lower frame rate than Angoville.
Do splines affect system performance?
Grass should be used to add details, not to use to cover large areas, like fields. Taking a look at Relic maps will give a good idea of how much grass should be used.
Here's a snippet from Relic's official mapping contest rules:
You can view render calls by clicking Advanced --> StatGraph. Render call is 4th one from the bottom, "render()", with bright red color.
6. All maps must adhere to the following design rules:
a) Building sets are limited to 1 (one) building set per map (e.g. Rural, Urban, or Industrial.
b) Do not use decals.
c) Keep render calls at around 300.
d) Try to limit water to 1 (one) water source.
e) Keep maps light. Too much variety in textures and objects will use up too much video memory.