The text is unclear, but the intent is to limit it to 1 Loresheet.
As you note, the V5 Core Book (as of the V5 Rules Errata 1.0) doesn't really make it clear that the intent is to limit it to one Loresheet per character.
In the Character Creation section, the "Choose a Loresheet" subsection states (p. 151; emphasis mine):
Loresheets are a special type of Advantage that connects your character straight into the rich metaplot(s) of Vampire. Each player may choose a Loresheet and select dots in it just like any other Advantage. Beware, however, that these Advantages are often very different in nature and may not be appropriate for all chronicles. [...]
Loresheets are further described in a subsection of the Advantages section (p. 190; emphasis mine):
Buy an Advantage from a Loresheet and integrate the story of that lore into your character’s lineage and connections. Some Loresheets (especially the “Descendant of ...”) require the character be of a specific clan.
Remember, that just like other Advantages, each level of a Loresheet is self-contained and must be bought separately. It does not automatically convey the “lower” levels of that Loresheet.
The Loresheets themselves appear on p. 382-396 of the book. There's no description of how they work in general there - only the lore and specific Advantages associated with each.
There's a final mention of Loresheets in "Appendix II: Projects", in the context of mechanically representing the goal of a project (p. 417):
To launch a project, the player (or players) must first describe their goal in story terms: [...] uncover the meaning of a mysterious glyph spray-painted on the wall of their haven.
Then, the player and Storyteller need to come up with the mechanical representation of their goal. This usually takes the form of one or more dots in a Background or other Trait. [...] figuring out the glyph might provide dots on a Loresheet or allow the purchase of a Blood Sorcery Ritual.
As you've mentioned, none of this clearly indicates that players are meant to be limited to a single Loresheet for their character. It could just as easily be interpreted as explaining the procedure for choosing any number of Loresheets and selecting dots in them in general, rather than restricting it to a single Loresheet.
However, Reddit user /u/elmerg notes in this comment thread on /r/WhiteWolfRPG that Karim Muammar (chief editor at White Wolf Publishing and then at Paradox Interactive after they restructured/integrated WW) had clarified that it was indeed meant to be limited to one. It was clarified in a Facebook post that was compiled into this FAQ (by the World of Darkness 5th edition Discord server):
Q: How many Loresheets can you purchase?
A: One. The wording on pg. 151 and 190 cites ‘a Loresheet’. This was also confirmed by Karim.
As you can see from the above screenshots, the intent is to limit it to one Loresheet per character. This is a valid interpretation of the phrasing in the rules, albeit not an obvious one.
Hopefully a future errata of the Core Book clarifies this limit in the text itself, rather than relying on clarifications of intent in Facebook posts. At the least, it's good to see it clarified in upcoming books, though such rules should be clearly stated alongside the sections where they're introduced in the Core Book.