If it is possible to simulate a universe such as ours, and there definitely is a real universe somewhere (which is an entirely different can of philosophical worms) then it is most likely that there is more than one simulated universe being run in that real universe. In fact there could be a great many simulations running right now (either due to one civilisation at the relevant technological level running many simulations, many civilisations running one each, or many running many) and that multiplies up over the course of the real universe's remaining existence.
We are either in the one real universe or one of the many simulated ones, so it is more likely that we are in one of the latter.
Even if you consider the possibility of there being an infinite number of real universes this basic arithmetic still holds: for each real universe there is potentially a great many simulated ones.
To compound it further: if the simulation is so detailed that it is possible for the simulated life to perform its own simulations, then each simulated universe may contain many more simulated realities. Turtles! All the way down! There will be a limit to how far this can go of course as each simulation must logically be simpler than the parent reality (caveat: some might question if that logic is true or just a limitation of our current knowledge about information theory) so the complexity of the real universe(s) will dictate how may levels of virtualisation we could potentially find ourselves within.
I got into an argument with Kurzweil on his blog once years ago regarding the details.
The simulation does not necessarily need to be simpler, subject to a few constraints: It needs to be possible to represent the simulation in less space than its containing universe. It also needs to be possible to execute the simulation in the available space/energy, but if acceptable it may be sufficient to be able to execute it at reduced speed which can substantially reduce the amount of matter/energy required at any one point as you'd be able to e.g. more aggressively apply compression etc. to the data.
The space requirement can be fairly small depending on the constraints on your simulation. E.g. are you simulating a whole universe? A planet? One person? Everything not directly observed by a simulation subject can be aggressively pruned as long as it can be reconstructed in a believable state (unless you are trying to accurately simulate the physics of everything), and even things that are observed can be simplified dramatically. E.g. in almost all cases nobody would be able to tell a rough approximation of the physics and appearance of cloud formations from "the real thing", so you don't need to simulate them particle by particle. On the other hand, one such optimization/approximation then could be to fake the result of any attempt to run your own simulation.
One thing I find interesting is that Bostrom talks about "ancestor simulations" a lot, but there's no reason to assume that if we're in a simulation it has to be a simulation of the past of whomever runs the simulation - it could be a game, or someone simulating aliens (to them), or it could even be a civilization that decided to create themselves a nice simulation to live in, or a "theme park"/"zoo".
We are either in the one real universe or one of the many simulated ones, so it is more likely that we are in one of the latter.
Even if you consider the possibility of there being an infinite number of real universes this basic arithmetic still holds: for each real universe there is potentially a great many simulated ones.
To compound it further: if the simulation is so detailed that it is possible for the simulated life to perform its own simulations, then each simulated universe may contain many more simulated realities. Turtles! All the way down! There will be a limit to how far this can go of course as each simulation must logically be simpler than the parent reality (caveat: some might question if that logic is true or just a limitation of our current knowledge about information theory) so the complexity of the real universe(s) will dictate how may levels of virtualisation we could potentially find ourselves within.