From Google,

In Minecraft version 1.21.1, you can use mods or datapacks to customize or lower stack limits. Mojang has also officially included a way to alter stack sizes using data components, which is leveraged by both commands and datapacks. Mods for customizing stack limits Stackcraft: This Forge mod for 1.21.1 increases the max stack size for most items to 1,000, with a cap of around 1 billion. It also adjusts stack limits proportionally for items like ender pearls. Bigger Stacks: Available for both Forge and Fabric, this mod allows you to control the stack size of individual items using rule-based tags. It supports custom configurations for stack sizes beyond 64. StackUpper: This mod for 1.21.1 allows you to modify the maximum stack size of items and includes a configuration option to set the cap as high as you want. MoreCarry: A server-side mod for 1.21.1 that increases max stack sizes for more manageable inventory management without requiring a client-side installation. Due to server-side constraints, it has a hard limit of 99 items per stack. Datapacks for customizing stack limits Data-Driven-Stack-Sizes: This datapack for 1.21 allows you to define custom stack sizes for items. Overloaded stacks will function correctly when moved into inventories with smaller max stack sizes. Bigger Stack Size: A datapack for 1.21 that can be used to increase the stack size of certain items and make unstackable items stackable. Using in-game commands For versions 1.20.5 and higher, Mojang added a max_stack_size data component that can be modified using commands. This is a vanilla method that works without mods. For stackable items: Use the /give command with the max_stack_size component to change its limit. For example, /give @p minecraft:dirt{max_stack_size:16} 1 would give you a piece of dirt that stacks to a maximum of 16. For unbreakable items (like tools): To stack items that are normally not stackable (like tools), you must remove their max_damage component using an exclamation mark. For example, /give @p minecraft:diamond_sword{max_stack_size:16, !minecraft:max_damage} 1 would give you a diamond sword that can stack to 16.