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Not necessarily, sometimes you have to see how many moles are needed in the balanced reaction. For example, if 4 moles of substance A is needed while only 2 moles of substance B is needed for a reaction, but you have 7 moles of A and 5 moles of B, substance A is your limiting reagent even though there are more moles of it.
The smaller amount of moles is limiting but you must consider the stoichiometric coefficients on the reactants side. If the ratio of coefficients is not one to one then the smaller amount of moles calculated from masses given may not actually be limiting.
The limiting reactant is easiest to understand if you think about it as the first thing that "runs out" in terms of number of moles. When comparing the necessary amount of moles for the reaction, the one reactant that limits the product mass is the one that is used up/runs out completely.
To help further illustrate this, one of the examples that Dr. Lavelle used in class compared a reaction to baking brownies. If you have a lot of flour, sugar, and chocolate but only a few eggs, the amount of eggs will limit the amount of brownies you can make. This is the same for a chemical reaction. If you have excess of one reactant, but only a little bit of another, the one that there is not enough of will limit the amount of product you can produce. This does not necessarily mean that the one that has fewer moles always limits the reaction, the reaction is limited by the reactant that produces a smaller amount of product.
Not every time. It may be appealing to assume the smallest number is the limiting reactant but that is not always the case. For example let's say you've gotten to the part where you figure out the quantities of both. Lat's say you get 1mol of O2 and 0.25 mol of C3H8. One would assume that O2 is the limiting reactant but when you plug in all the numbers, you end up getting 1.25 moles of 02 that are needed to react with 0.25 mol of C3H8 (propane). You however only have 1mol of 02 meaning you do not have enough 02 to react with the 0.25 mol of C3H8 making 02 your limiting reactant
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