Is there a limit to top dress plants with compost? When I top dress my fruit trees and flower plants and shrubs with compost, how much should I top dress? 6 inches enough? or it does not matter. I can top dress as much as I want given how much compost I can afford?

Thanks

I usually spread a few inches over the entire area.
Six inches?
I've never had enough to spread it that thickly...
Although...
One time I advised a woman to cover her corner lot with woodchips to kill the turf... She had like an acre and a half... Nobody needs that much turf...
She spread it like a foot + thick... Gorgeous!
I wanted to plant some flowering plants in her yard, but she was happy with the empty look.

Will the layer be so thick and heavy as to limit oxygen getting to the roots
?
And is the stuff rich enough that it adds too much nutrient all at once?

3 inches seems safe to me, 6 inches makes me nervous.

"Compost" is not an exact thing. If you're talking about something purchased in a bag, since you mentioned "afford," the one time I got a bag like that, it was barely distinguishable from "dirt." Not something I would trade for $ again.

Or are you using compost from a home pile or bin?

If you don't have a serious weed problem trying to get rid of (I wouldn't suggest compost in that instance anyway), then 2-3 inches should suffice.
Coarser materials (like wood chips) can be laid down thicker since they allow better gas exchange due to their pore sizes. They don't go hydrophobic in dry conditions either, something compost tends to do...
If the bed you're trying to mulch is ornamental, I highly recommend the latter. It decomposes slower and has overall greater benefits to the soil and plants.