Is ablution and prayer valid if I doubted the number of times I washed my right foot, and I washed my left foot twice, given the permissibility of washing some parts of the body for ablution a different number of times than other parts?

If you doubt the number of times you have washed a limb during wudu', then build upon certainty, which is the lesser amount. This means, if you doubt whether you washed your right foot once or twice, consider it as once, then add to it to complete the required or preferred number. Wudu' is not invalidated if the number of washes differs between limbs, such as washing some once, some twice, and some thrice. Wudu' is valid in all these cases because what is obligatory is to wash once, and three times is better, and varying the number of washes is permissible.