There are no general rules limiting the number of spell slots. The Spellcasting rules only say:
Thus, each spellcasting class's description... includes a table showing how many spell slots of each spell level a character can use at each character level.
This can't be a hard limit on usable spell slots, as you can gain additional spell slots with features such as the Boon of High Magic from the Dungeon Master's Guide:
You gain one 9th-level spell slot, provided that you already have one.
The wizard's Arcane Recovery says (emphasis mine)
Once per day when you finish a short rest, you can choose expended spell slots to recover.
These must be expended slots, but Flexible Casting says:
You can transform unexpended sorcery points into one spell slot as a bonus action on your turn... creating a spell slot of a given level. You can create spell slots...
Any spell slot you create...
All the references to creating slots shows that these slots need not be expended, and since there is no hard limit on the number of spell slots a character can have, a sorcerer can stack up additional spell slots beyond their level's baseline. This includes spell slots of a level beyond those usually available to a spellcaster.
Jeremy Crawford, lead rules designer, supports this interpretation on Twitter:
Using Flexible Casting, a sorcerer can convert sorcery points into spell slots. The number of sorcery points you have is the only limit on the number of slots you can create in this way. (Remember that the slots go away when you finish a long rest.)