Well there are 2 things to care about, first is map loading time, it's always better to use loop or timer to create those doodads after map start to reduce loading time.
2nd thing is related to lag, some spells use 50 or more dummy units at same time and still perform flawlessly. The thing is you need to care about unit/doodad animations, if unit has complex animations it will reduce FPS faster, for example if you use UNDEAD LOCUST as test model it will reduce FPS (frames per sec) from 64 (Normal) to 20 (game perform slower below will lag or freeze screen) already after 500 units show on the screen at same time, but if you use HUMAN BANNER FLAG you will need 700 units for same effect. Model without animations like BENCH needed 1000 dummy units to reduce FPS from 64 to 20.
Remember FPS is related to SHOWN SCREEN, so it's LAG as well.
If you use many animated doodads in your maps and huge sigh rate (disabled fog on screen) then reduce animated objects.
If you use fogged terrain with hidden objects like units and animated doodads then don't worry about lag.
Oh I ignored fail triggering in my calculations, also I made this test on 100% empty terrain, doodads and units will reduce values from above by 20-50%...