Is the US aircraft limited or base limited?The difference is that the VKS is quite a bit smaller fleet wise and cannot effectively fly beyond the line of contact aside with the occasional Su-57 deployment. The USAF is already flying over most of Iran and that's with a portion of their total fleet, as many assets are still stationed in Europe, East and South East Asia and the United States itself.
And even then, Russias superior industrial output, bombing and missile campaign against Ukrainian industry and energy has ultimately given them a decisive drone edge at the front (as admitted by Ukrainians themselves) and especially in the long range drone realm. The USAF however could do what Russian Su-34s do at the frontline with their FABs, just at a much larger scale across the entire country.
It's worth keeping in mind that the Americans so far are reluctant to fully commit due to domestic backlash. Their deployed capability isn't their theoretical capability.
Inside Russia for Russian planes there is essentially infinite airfield capacity.
Is there infinite capacity for hosting US planes in the Middle East?
Where do you think US planes are being hosted from? How many air refueling do they need to reach the battlefield? Russia needs 0.