To simplify things: You can switch weapons once during your turn.
Unless that weapon has a requirement such as the Chained Stiletto.
Which requires either an action or bonus action.

Normally we won't bother too much with switching weapons mechanics. We ignore standard loading/ammunition just because it's a hassle to deal with that.
Some DM's argue that during the first turn after rolling for initiative, you must declare that you 'draw your weapon'. Or you can't use it on turn 1.
All this above just sounds like overcomplicating the game to its most boring elements. We're here to have fun, don't we?

Officially speaking during the “Free Action" you can only draw or sheathe a weapon, because that would be two object interactions.

2014 Player Handbook: You can also interact with one object or feature of the environment for free, during either your move or your action. For example, you could open a door during your move as you stride toward a foe, or you could draw your weapon as part of the same action you use to attack. If you want to interact with a second object, you need to use your action. Some magic items and other special objects always require an action to use, as stated in their descriptions.

To simplify things: You can switch weapons once during your turn.
Unless that weapon has a requirement such as the Chained Stiletto.
Which requires either an action or bonus action.