Function getGridColumns

Calculate the optimal number of grid columns based on participant count.

Thresholds:

• 1 → 1 column
• 2 → 2 columns
• 3-4 → 2 columns
• 5-6 → 3 columns
• 7-9 → 3 columns
• 10-12 → 4 columns
• 13+ → √n (rounded up)