Hi Bob, it depends whether you want large leaves, in which case grow single plants, or more, smaller leaves which grow well from clumps of two, three or four plants.
Chard seed is cheap, you can sow three or four seeds per cell and thin to however many plants you want, of the colours you want too if it is rainbow chard.
One tip, if you sow now the plants will probably bolt in June. A second sowing in late April is good for the summer and autumn.