Most are solitary
The safe default is one tarantula per enclosure. They do not need a companion for social welfare.
Cohabitation adds risk
Two spiders can compete for retreats and food, injure one another or cannibalise. A setup that appears peaceful for weeks can still fail later.
Communal-labelled species
A small number of species are discussed in the hobby as potentially communal, but that is specialist husbandry with real risk, not a reason to house ordinary species together.
Communal safety is species-specific
Temporary tolerance among tiny siblings does not prove that long-term cohabitation will work as they grow.
For a beginner considering communal keeping, start with the species and its documented social tolerance. In do tarantulas need companions, the priorities are a correctly identified species, secure housing, appropriate substrate or climbing structure, a retreat, water, ventilation and prey. Extra equipment should solve a specific problem rather than create one. Write down the scientific name and recent moult or feeding information, and learn what normal behaviour looks like for that animal before reacting to every period of hiding, fasting or digging.
No benefit for beginners
A new keeper gains nothing by turning a straightforward single-spider enclosure into a group experiment.
One spider, one enclosure
Separate housing makes feeding, moulting, health monitoring and escape prevention much easier. It remains the standard recommendation for almost all tarantulas.